I'll be honest, it really kinda annoys me that people try to make an actual living re-selling old video games. That's not a real career. It just isn't. It's a tiny market, it's really scarce, and it's all riding on nostalgia and trendiness with a ticking clock and it will burst. But, undoubtedly these types will move on to the next hot thing. Holding attic trinkets that people want at a ransom, is just a pathetic way of putting food on the table.
I've said before, I know a really honest reseller. Not all of them are jerk-wads who just hunt out video games only and look them up online. Some just want a small markup from what they bought it at, and don't care what the game is. Unfortunately, not usually the case, but sometimes. If you're gonna flip yard sale junk, don't gouge people. I think it's that simple. Price things reasonable and you'll sell more, and make more.