These bods need to be called on these things. As soon as the ad appears, ring and ask to inspect the bike. Take cash with you and if all seems ok then pay and take the bike there and then. It is quite possible that they are monitoring this and other sites so that if it all becomes much talked about they scurry off. With todays instant on the spot communication numbers can be checked there and then. It is possible that a bike can be stolen. Get them to sign applicable paper work, take photos, lots of them not only of the bike but of the seller and his surroundings and other vehicles. Take some one with you, they can be observing whilst your talking. The sooner dodgey sellers realise this is the scrutiny they are up against the more they will think about dodgey selling.