Did the seller represent that unit as occupied and income producing?
Charles is much more experienced than me, but personally I disagree a bit, I think that’s pretty bad on the seller’s end. Can you get a police report or police records proving that the seller was notified of the issue?
If you can, I would approach the seller very nicely about the subject, assuming it was an oversight and not nefarious, and discuss potential compensation for his misrepresentation. If he balks, I’d then lay it out in black and white: sending him the sales contract that (I’m assuming) said he would be honest, the rent roll he sent you, and then the police report proving that his rent roll was wrong, and that he knew it was wrong. I’d then send this to him, and follow it up with a letter from an attorney. If he still balks, it may or may not be worth pursuing further.