Dream Merchant was right - I would have advised the same thing. Go and document the infringing use of your works in the shop. Evidence gathering is the most important leverage you will have later on.
Once that is settled, my suggestion for this sort of situation is for you to tell him that yes, you can negotiate payment, but so long as both has not agreed on anything, use of the work remains infringing. "Advise" him to take it down until settlement is concluded - this will give him a time pressure not to drag negotiations.
Also, there is nothing wrong with identifying his shop here on the forum. In fact, that is another leverage you can use - inform him that if he can't settle within X days, you will be posting the identity of his shop on internet forums.
All of these is subject to you being able to prove what you narrated in this thread - after all, when you identify a person, you should be sure you don't end up being sued for defamation
Hope this helps. Let us know what happens next.