A good portion probably tell the truth (also who doesn’t look on the web to check anyway before buying) but a bargain is a bargain despite the “damp” problem. I once saw a ground floor flat next to the river in Hampton Wick that had no problem at all with flooding. It was surprisingly cheap and for some reason had all sockets half way up the wall and the bedroom had a raised platform to put the bed on, limited headroom but less chance of a wet arse when it flooded (which it did every other year).
Someone else took it. Good location, great view, cheap. They didn’t care that they’d be underwater every couple of years