how much do you make selling skills on marketplace, just curious? It seems like it'd be impossible to vet a skill without reading it, so the entire concept mystifies me just a little bit.

Usually only charge a few bucks because honestly, anyone can make them but those that buy get them because they don't want to spend the time on making them. You can sell them for $10 or $15, maybe more depending on the skill. Not always a lot but if you have a ton of them, you can make some serious cash.