Money Laundering was my first suspicion, or offloading fake notes.
Pay in 500 fakes and it’ll be you that’s committed the crime. Even worse is he hands over a wodge of fakes in person, and takes the bike before you can attempt to offload dodgy notes.
If someone wants to pay me cash, and I don’t know them, we go to the cash machine and I watch and make sure there’s no switch between the machine spitting out the notes and the notes entering my hand, and we do this in full view of cctv.
I know not everyone is a fraudster, but if I don’t know them personally, then on eBay etc. I assume they all are (seller or buyer).