For £20, probably not stolen. There's easier ways of scamming twenty quid than nicking a bike and going to the trouble of listing it on the 'Bay. Like a bit of shoplifting followed by flogging the haul down the pub no questions asked. Loads of meat and chocolate goes AWOL out of the local Sainsbury's this way. I've bought two bikes at that price or less and was 100% satisfied the sellers were kosher. I've known enough rogues in my time to know the difference between a genuine seller and a toerag punting out bent gear. The sort of premises you have to collect an item from is a dead giveaway for starters.