I like how eStarland does their trades. They offer different amounts depending on how complete the game is when you trade it. Plus, when you go to buy a used game they specifically tell you what's included with it as well (case, manual, extras, etc).
I might get ripped for this but GS should enter into the mainstream retro game market. Start taking in select games for all consoles... no sports games, no trash games, focus on the major franchises (Marios and Sonics) and releases in popular categories (RPGs/action adventure games for any 90s systems, racing/FPSes in the late 90s/00s). Train employees to actually review the discs and contents for physical quality before accept trade in (vs taking in anything and destroying them with your shit machines), and then just price them just like eBay.
People might not like hearing this but eBay prices are already nuts. If I have to pay $100 for an old game I want, I'd be more inclined to do it at GS where I can review everything vs eBay where the seller calls their water damaged/dog chewed copy mint condition.