Typically you won’t have real people harvesting emails for sale - it’s much easier to get addresses in bulk by hacking services and dumping their databases - and you get password hashes as well, which makes it a much better value proposition for the time it takes.
Off the top of my head, a couple of things that it could be:
Simple stolen card fraud - buy something small with a stolen credit card to prove it works before using it for a higher value purchase. Something where you don’t need to worry about a physical product getting delivered somewhere would be great for this
Escrow fee fraud - commission something small, then use that as a hook to promise a much larger commission, except that they want to use some off-site service to “reduce the fees”, then "oops the money has been flagged as maybe fraudulent and you need to pay a fee to get it released"
Money mule recruitment - again, small commission (using a stolen card) to establish a relationship, then come to you with a great job opportunity where they will send you some money via western union or something, and all you need to do is send it on to someone else, and you get to keep a cut. The catch is you are actually handing stolen funds or helping someone evade international sanctions, and you end up in prison