The first thing I do when I see a plane for sale is look up the tail # in the government data base. If all looks fine, I check out the owner on Google. After this, if I was still interested I would call the owner for questions. One can also order the disk on the history of the plane. And one needs to be aware, not all repairs may not be logged into the log book. Talking to the mechanic from the last annual or a number of annuals previously can help. Also talking to the airport pilots in the area could give some advice. I think some may appear very cautious as selling planes, can be like horse trading and not all is known about the plane or horse and the seller may not be willing to reveal things that should be revealed? In this game, a number of pilots have really gotten burned on a plane that was represented inaccurately. I do not understand how a plane can pass an annual before sale and then when the buyer has the next annual, there are many things that should have been addressed by the seller in the annual before the plane was bought.
