"On the Gumtree site,..."

This reminds me of a girl I knew in college. A big part of her payments were made by a dude, in exchange she sent him used socks/underthings, etc. She told me that, without fail, whenever anyone found out about this arrangement they were possessed of a bizarre need to attempt to exploit it even more. Like, she should send him stuff she'd bought in a store and never worn, or she should threaten to expose him and demand more money.

She never did. But I suspect that if she had, it wouldn't have been as big of a win as mostly free college. Win ++ would have been win --, because if someone's notion of the payoff matrix for cooperate/cooperate is 5 to you, 0 to them, don't defect.

Never forget that the subs are in charge. It's a paradox, but it's true. There are more ladies willing to live rent free than there are dudes with available property. If you tell him your plan up front he'll choose another. If you pull a bait and switch it might just not work (you made a "by law" statement, are you a lawyer? If not, run it by them before you do this). If it does work, it is likely that your relationship with dude is now, shall we say... icy? What might he do in this situation? In your model of the future, how many months do you get these rent checks before the state changes, and is that worth whatever the blowback is?