I did a pre-buy once, with the current owner of the aircraft hovering around, looking pretty axious. When he said that I sure did take his plane apart, further than it had ever been, I had a pretty good idea at what I would find. And I did. And I reported it to my boss, the buyer.
To make it work properly for everyone, a third party annual would be best. A pre-buy is equivalent to kicking the tires on a used car lot. It won't tell you much, other than there is (isn't) air in the tires. By splitting the costs of the annual, both the buyer and seller would be protected. The buyer, by ensuring he is getting what he is paying for. And the seller, by weeding out those who are less than willing to make a commitment to the purchase. My opinion, for what it's worth.
That being said, I did have a guy look at my project before I bought it. More to make sure it was all there and wasn't a pile of rust! He said he'd buy it if i didn't! Nuff said for me!
