[smnrchrds]

It's hard for the plaintiff to prove, but it's also hard to for the defendant to disprove without committing perjury. Keep in mind that in a civil lawsuit, the case is decided on balance of probabilities and there is no assumption in favour of either side. Also, there is no right to silence.

[TacticalCoder]

What I take from this is that if you want to rent your house to someone and don't want to sell it and certainly don't want the renter to lie about it and say you promised to sell the house, you then have to specify in the renting contract that there will be no selling of the house to that renter. This greatly lowers the probability that you then changed your mind and somehow promised orally to sell it.