Ye Olde Inn

Including all the expansions, there are a ton of artifacts in Heroquest! What makes them special is they cannot be purchased in the armory. But if you did put a gold coin value on them, I'm curious what value you would attribute to them.

For example, have you played in a game where players bartered and/or sold off some of their artifacts to the other players?

Nope. Each artefact is a unique item.
There's no selling from hero to hero, only the card can be given to another hero.
Mostly, the group in these cases determines which hero should have the item for tactical or just restrictional reasons.
I make it very clear to my heroes from the start that there is no price on an artefact.

Of course, unless I say so. There might be a backstabbing way for me to take one from them by having an NPC in the dungeon paying good money...
but that will be a spontaneously assigned sum. Or some other item to be swap with, like a weapon or a lost artefact that will soon be needed. Deus Ex Machina.

My current players have never sold stuff to each other. If one has a thing that is better served with another hero they just pass it along.
Now I think the lesser artifacts like the scroll spells in expansions should have a value similar to the values we see with potions.
For the really awesome things like Borins Armor, Spirit Blade and so forth...
yikes...there value would be near incalculable in such a setting as Glorantha. But we do tend to set 'Gold Piece" values to almost everything in these games.
I'd say if you were a hero coming to my Dwarven Smiths armory and you wanted a suit of Borins style armor I'd have to charge at least 10,000 gold. If you wanted me to buy it from you..."I can put it up for auction..." and then maybe the hero gets 1/3 of the value of the item.
Simple things that just recreate a one shot spell in some way would be much cheaper than an item that has a durable power.

Some things like Rabbit Boots although they have a durable enchant would not fetch much. But would cost a pretty penny to procure.

Mainly this is a non issue for my table. You play a base hero who is issued a pile of gear from the Empire, so that you can complete the mission. The players must divvy up the items between themselves. All weapons, artifacts and armor are surrendered at the end of the quest to the Emperors armory. Next time the Emperor sends you on a mission if the level isn't that difficult then the issue of weapons and magic items will be less.

In my ruleset, Artifacts are divided into several subcategories: Spell Scrolls, Magic Items and Artifacts. The former two categories have a gold coin value and can be bought and sold at a Reliquary, while full-bodied Artifacts cannot. The difference between a Magic Item and an Artifact is somewhat arbitrary: one-use-only or limited-durability items such as Magical Throwing Daggers, Holy Water, Fire Rings, etc. tend to be classed as Magic Items, whereas enchanted weaponry, armor, or multiple-use baubles such as the Spirit Blade, Borin's Armor, the Armband of Healing, etc. are usually Artifacts.

While my system's Artifacts cannot be sold, they do have an auxiliary use: In the event that one or more Heroes die during a Quest, Mentor is willing to take one or two Artifacts as payment in lieu of an Elixir of Life to resurrect that Hero at the end of the Quest. (That particular topic should go into its own thread.)

Usually the Magic Items are too useful to the Heroes to sell, though, so I've never seen that happen (although there was one memorable incident with the alternative party's Alchemist where he managed to make way too many Potions and sold a few of the excess for over 1,000 gold). Buying them is a little more common, but not by much, mostly because it's very difficult to convince the Heroes to spend their post-weaponry gold on anything other than Potions of Healing without making them heck of expensive. For reference, I currently have the Potions priced at 300 gold each, while some of the other Magic Items cost:

Anti-Poison Quill: 200
Dust of Disappearance: 150
Fire Ring: 250
Holy Water: 400
Magical Throwing Dagger: 400
Ring of Return: 400

...Something tells me I need to work on rebalancing the economy a little better.

On reflection it seems to me that with enough coin a hero should be able to buy anything.
If he buys up a pile of Mithril and takes it to the Mage Smith he should be able to buy a Borins armor copy.

My heroes players are pretty mellow people but I can see them getting really irate with Mentor and them demanding an opportunity for combat if he tried to milk them hard for an Elixer of Life. They would be "WTF Old man. You sit in this tower and play god with lives, while you are not healthy enough to go questing....You better come down with these prices or there will be troubles....!"
Then if they got into battle with the arch mage and I used "DM powers" to rule they loose, it would be the end of games with this crowd. They would demand a chance at fair combat. Then I would be without an arch mage and they would loot his library for sure. LoreTome is right there....
Yeah it could get very messy.
Sure the arch mage can get off maybe two spells and nuke two heroes but after that....dead Mentor. And if he was powerful enough to survive a sudden combat due to an escalated argument with the four heroes, the players would call cheese on me all day long. "Why the old dude could have gone into those dungeons on his own and dealt with chaos single handedly. Why did he need heroes to do his bidding. You need a better storyline and better NPC's."

If such things could be purchased at all, how much would a proper Elixir of Life cost in gold coins? Would 3,000 be too outrageous... or not outrageous enough???

3,000 would be my price for Borin's Armor.
Elixir of Life? The reason why I don't tag coin to artefacts.

From Gameplay point of view, it's not worth a lot. If the heroes are clever enough to keep the stuff of their dead comrade and to give it to the "new" replacement Hero then the elixir of life that you buy inbetween Quests to relife a hero has no real ingame effect at all.

If you give them one in advance, that they can use immediately that someone dies, it is worth at least 2 or 3 healing potions from Bodypoint of view. From ingame effect, if the balance is for the heroes when they are four, and against the heroes when there are just three left, it is worth maybe double, so 4-6 healing potions.

Now from RPG point of view it must be much more. If you allow the Heaven to have a revolving door, death loses it's effect and is "just another thing that happens from time to time". Heroes might even develop ideas to kill one of them, ask Ragnar some questions and get him back via Elexir of Life. This is cool if it results in an extra Quest with Ghosts and Devils and Angels, but if it becomes the standard procedure, death is becoming boring; and as a result the quests might lose their tension. I don't know, maybe they work in spite of revolving heaven's doors; decent seems to work also and there no Hero ever dies - but i think it adds to the game if you think "oh no that room full of monsters over there could be my end".

So I would say Mentor can't produce Elixir of Life in masses. Even he should have only one or so; to produce it, it shall be necessary to have the entire blood of five virgins of high noble rank injected into a living red dragon (anything that is necessary so that he hasn't the idea to produce more); and the other way round i would send the heroes from time to time to get one to Mentor (so he doesn't have to sacrify five virgins). And if the Heroes want one from him, they shall give him two or three unreplaceable artifacts. They can subsequently lease them back for 100 Gold per Quest or more. (no that was a joke )

In Super Mario Bros., an extra life only costs 100 gold coins

So long as Mentor had a potion on hand and he refused to give it to the Heroes due to cost or difficulty procuring ingredients and if the heroes knew it.....It would be on. They would not care about saving their skin at that point and they would take the Elixir from him by force. Now if there was no potion on hand and they had to quest for stuff to get the potion...they would do so.
But the second I gave them any speech about how the elixir is for 'the royal family' or how the dead guy doesn't deserve it, I know my players, they would be incensed and outraged at this supposed good guy. He would have to die. And honestly...keeping harmony at my game table is far more valuable to me as a GM than any NPC. No NPC is so all important to storyline that they are untouchable.