It's a peer-to-peer agreement (both are agreeing to a contract to swap seats for some form of consideration). I post that my seat is available. Someone offers consideration for that seat (and their own seat on the same flight). We agree to swap. We board normally but sit in the other person's seat. The platform exchanges the consideration.

As for how airlines could try to prevent it, yes, we've considered it and have a few different approaches. Worst case, the contract can be voided by agreement of the parties and everyone sits in their originally assigned seats.