Can a tenant sue for a bad-faith N12 eviction if the new owner doesn't actually move in?

Yes. The Residential Tenancies Act, 2006 allows a tenant to apply to the Landlord and Tenant Board for a remedy where they believe a purchaser's or landlord's own-use notice was not given in good faith, meaning the person who claimed they needed the unit never actually genuinely intended to move in, or did not follow through. If the Board finds bad faith, the consequences for the person who gave the notice can be significant.

This matters directly for a new owner who used this kind of notice to gain vacant possession after a purchase, since the genuineness of the intention to occupy is exactly what can be scrutinized after the fact, not just what was said at the time. If your plans change after obtaining vacant possession this way, understand that a former tenant may still be able to challenge what happened, and the outcome depends on the specific facts about your actual intentions and conduct. Anyone using this process, whether the original owner or a purchaser, should be prepared to demonstrate a genuine intention to occupy, since this is squarely what a bad-faith claim puts in question.