I don't believe they do. I don't know any chain that does. Maybee Blockbuster and Hollywood video because they are new to the game but thats it. Tell all phone callers something generic. We tell them "we pay out based on demand,condition,rarity,# in stock at the time you trade it in." if they push on old stuff "We are the only store both giving trade or paying cash on those items. If you want to bring them by we will be happy to look at them and make you an offer to give you the best offer we can."
Most chains don't take SNES anyway. These people will often take 10 cents a game or less if they are cleaning house. You can get away with paying less on stuff no one else will take. You are the only game in town.
I use this as a gauge. If you don't have one person turning down your offers you are paying too much.