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.