The company replied to my concerns on Facebook.
They said they know old sports games aren't coveted and they don't even deal with them. They also assured you will never get duplicate games, because they keep a database of all users and what games they sent. Also, they said you can check off what systems you own, so you don't get sent useless games. And finally, they assured that they are trying to keep a good rounded amount of action, RPG, adventure, platformers, all genres.
So it sounds promising. But they did say that due to the nature of old games, they can't ensure that they will come complete or with manuals. They did say that they guarantee they will work and they will have labels fully intact. Personally I've never been a completionist collector, so that doesn't bother me. I collect to play.