Ordinarily sellers disclaim all liability; it's called "contracting out" of liability and it is legal as long as the disclaimer language is bold and all caps (it has to stand out from the rest of the contract).
However, this guy is also the builder, so he may have to beef up his disclaimer - but as long as he does that, he should be protected. In other words, the parts that he bought are usually warranted from the manufacturer and he should pass on those warranties. For example, the Lycoming engine probably has a 20 year warranty and that warranty should be passed on. In the event the buyer crashes and it is due to a defective engine, then he can go after Lycoming rather than the builder/seller.
That's why, when you sell your home built, make it clear in the contract that the buyer knew he was buying a home built aircraft. That way, he cannot come back and say he did not know it was not designed by FAA approved designers.