I try to keep on top of going rates (If there really is such a thing) of goods that I search for at thrift shops. So it truly depends on what I see. I see Laserdisc movies for a couple bucks each, I'll gladly pay the price they ask and just move on knowing I've made a great deal. No point in trying to milk a few extra dollars out of the sellers. However, if I see something drastically overpriced I usually scoff at it or sometimes mention to the clerks how overpriced they are. IE, once I saw NES SMB, Contra, and RC Pro Am for $20 each, loose no case or instructions. Wasn't even worth going up to the counter to let them know these weren't worth more than a few dollars each, as they just told me they would go down a couple dollars only.
It's like.... really?