Sorry for copying my own post but since it contained actual feedback on the mechanic and sadly was totally ignored, I felt it was kinda necessary:
I like the idea a lot but $3 is way too expensive for it as-is; (1) because it's weaker than any other trasher for $3 or weaker than almost any card for $3 in general at the beginning of the game, and (2) other players using different methods of trashing can join in late on the Thriftshop buisiness and take away all cards from the mat and spike a Province without having had to put up with a slow trasher.
Changing it to putting up to 2 cards on the mat seems okay but I think I'd also be interesting if the card cost $0. $2 would also work but for maximum fun you want like all the Thriftshops to be in people's decks as quickly as possible to make sure the mat content gets exchanged constantly. At least that's what I think would be fun.