I kind of feel the same way. If you don't know the value of things, don't sell them. It's not difficult with forums like this, Ebay completed items, etc, to find the value or an item. And expecting morality and fairness from a place that's going to re-sell your item to another person is moronic. My friend took his guitar and amp to GC for trade-in and was offered $75 for the guitar, and somewhere around $50 for the amp, if they even would take it. He sold them both on Craigslist a few days later for $800.
My old guitar teacher has a 60s Gretsch he got for $300 from a classified ad. He found an ad that read "gretch guitar," went to the house, and checked it out. It was an elderly woman selling it, it had been her husband's, and he had passed away. Perfect condition, worth probably 10x what she was asking. He took a few minutes to debate and told her, flat out, she could make a small fortune if she sold it to someone else. She said, "it was my husband's and he's gone, get it out of my house." $300 and he's got one of the nicest guitars I've ever seen or played.
Even if they got the guitars in there for nothing, they still ripped someone off. If nothing else, they stole from the shop owner. I agree that you should always do market research. Pawn shops, GC and the like are out to make a buck any way they can. Only expect about 1/10 of the value when dealing with places like these, but know the phucking value!
As far as the Gretch guitar teacher, at least he tried to help her out. So I don't see any fault there.