I've seen and read a couple of articles on this including one the problem was a lack of a free market.

Ultima Online tries something liek this and you would have things like player bakers undercutting the price offered the shops.

A couple of systems in NWN allowed players to create their own bot shops, but the problem I saw is that players would all too often abandon them.

I know we are talking SP here but economics is definitely a system that is important that you want to work right. In Arcade RPG's like Ghost's and Goblins you collect coins for shops at fixed levels. In diablo 2 the economy was crazy - but it was a different take where crafting became more important that the shop.