I think it's a really good idea in theory but I could see things getting complicated if for example if a hobbyist wanted to trade something pretty expensive that would take more than...let's say about three sessions to pay off. At what point does the item in question become property of the provider? If he gives it too soon he risks her just taking it and not fufilling the entirety of her obligation, however if the provider fufills any part of the obligation BEFORE recieving the barter item she is in risk of giving a free session(s). I guess people's reputations would come into play a little here but is a reputation enough when you are risking large sums of money?
Things could get complicated.