I have sold two amateur-built airplanes in the past, and I have three lawyers in the family, so I guess I have an opinion on the subject based upon actual experience and free professional advice.
Point one that has been made - You can be sued for any reason by anyone. Period. No escaping it. It doesn't matter what contract clauses you have, if someone wants to sue you, they can and will. of course, this is true today even without building an airplane. Forget to shovel the snow from your sidewalk and the pizza delivery guy falls? Guess what, you're liable.
Point two - just because they sue you doesn't mean you'll pay them anything. Deep pockets and tight contract clauses help to dissuade a lawsuit. As has been stated, if there's no money to be had, most lawyers won't pursue it. Setting up a corporation doesn't really protect you if you are the builder. You are still liable as long as the airplane is flying. If you are a buyer of an airplane, then the corporation can help to shield your assests, but they don't do anything for you as a builder.
Solution? Sell your airplane with a good, reasonable sales contract and don't worry about it. Unless someone can prove that you knowingly deceived the buyer about defects that you knew could cause an accident, and if you have enough money to make a lawsuit worthwhile after several years of litigation, then the likelihood of you being successfully sued is quite low.