The problem is that these retro game stores have to raise there prices to compete with ebay and amazon. If I owned a game store and you brought in a copy of say, Shining Force II, which has recently finished auctions on eBay for ~$35 both complete and loose. Lets say I sell the complete copy for $30 to have my rates be lower than eBay, then you bring in your copy and I offer say $18 to you in trade credit, since I need to make my cut to pay my overhead, why would you sell your game to me when you could list it on Amazon or eBay, and even after fees, get nearly $20 or so more for your game? See the problem here? Unless I charge more to offer you more, cutting into my own bottom line, what incentive do you have to sell your game to me? Of course, on the flipside, if I start chairing more then no one buys from me, making it a very delicate situation.

Sadly, retro game stores are a niche and dying market because of this, most collectors are more than aware of what they have a would much rather sell their stuff themselves then sell to a game store since they can get more for it that way.