Keep in mind that not everyone on eBay is a professional seller, or a game dealer, or someone who even knows anything about what you're talking about. Lots of people just list their games that have been lying around the house, sometimes rare games, sometimes sealed games too. Snap a photo with their phone and that's that. They don't realize they're appealing to a highly picky collector's market. Have you seen the ones who say something like - no instruction manual comes with it, but you don't need that to play the game!... Not everyone is a expert on this stuff.
I have some perfect condition sealed games, but I wouldn't think of listing them on eBay... sealed collecting is a whole other beast, something I def don't get into.
I've bought and sold on eBay many-a-time without much incident. Biggest issue is the way many choose to ship fragile, or otherwise expensive purchases. Bubble mailers do not cut it for Sega Saturn games, or even PS1 games if you want to be safe. I've had some occasional games show up and the booklet is way crusty and they didn't mention it. But, in most cases, I just cut my losses. When it's really expensive, I'll explain to them what happened and ask if they'd do small refund to curb the expense for being less than what I bargained for. But I wouldn't even bother sending it back.