When you sell artefacts in the vault you get credit BGs, that can only be used in the vault, for up to half the price of an item.
For example, if you want to buy a shield for 3,000 BGs. You have 4,000 BGs in credit, so you pay 1,500 real BGs and 1,500 credit BGs. Try clicking on an item, and you'll see the details of what you'd need to pay. The amount of credit BGs you have appears near the top of the vault window.
It's their way of making sure you can't get real life money by selling artifacts, and that you HAVE to pay real life money (or farmed bg's) in order to actually buy an artifact. One of the worst aspects of the game, really.