I'd guess that someone tried to steal the bike (possibly the vendor?). The bike may well be straight enough to ride perfectly well, until it breaks, which I wouldn't expect to be all that long. It's what looks like a crack round the base of the seat tube weld that's serious , rather than the mangled down tube.
It's basically worth what the parts on the frame are (brakes, wheels etc), and may well be worth £100 if they are in decent nick.