No!!!!!! If them damn vendors are going to charge me 2100 for a 650 gold piece item. Or offer me 75g for it, I am going to pick them clean. Cheap punks.
But honestly I think your idea of the shopkeeper making a perception check. They could "send guards" to search you if successful unless you can get away. I say disable fast travel when guards are in pursuit of you. But that would have to be like a 18-20 roll. Something high. Otherwise when you come back, they could be suspicious of you and only offer like 2g for a 650 gold item. 16-17. Etc etc etc.
