A thought, though it veers away from the core, allow shops to set up wagers on things - like success of quests, or who will be the first group to have a member die...
Visit the Inn after a hard day's work in the shop sort of thing 
Oh, I do have some evil thoughts. That favorite group of that other shop, the ones who spend so much money there and always go back, and now they are in your shop... sure, you can try to win them over with really good deals. Or you can talk them into crap and make them trade-in their cool stuff, because you know the quest is really dangerous and if you manage to strip them of this and that, a total party kill is absolutely possible...
