My brother selling it:
We had the ultimate NES collection as kids. Our step-grandma used to send us her NES games after she beat them, often with accompanying Nintendo Powers that had the guide to that game.
So by the time we had finished, we had every "must-have" in the library. Every Dragon Warrior, every Mario, both Zeldas, every Mega Man, all 3 Lolos, the Ultimas, all 3 Double Dragons, both DuckTales, every Castlevania, and so on and so forth. We had the official cleaning kit. We had the snap on thumbstick controller accessories, the full on arcade stick, the gun, Game Genie, the hard-plastic holders that hold like 16-catridges, hard individual cases for the cartridges, the manuals...the whole 9-yards. (The only thing we didn't have anymore were the original boxes, because as kids we didn't think to keep them.)
My brother sold it all to GameStop (right after they bought out FuncoLand, so they were taking classic trade-ins still). And what did he get out of it? A freaking Game Boy Color when they were still new, with a game carrying case, and a handful of games. It still haunts me. Luckily, most of the games I really liked I have either re-acquired, or for the more expensive ones they've been re-released (Dragon Warriors via GBC & DS, MM Anniv. Collection, DuckTales 2 recently, Ninja Gaiden 3 on Wii VC, etc).