Just so you know, I'm not a lawyer so this is just some friendly advice.
First get the contract in writing. Next include the language "AS IS" or "WITH ALL FAULTS." Make that language prominent on the contract (like a bolded heading with all caps or something along those lines). This gets rid of all implied warranties that are made.
Next, She just needs to NOT give any express warranties. Express warranties are where she would say verbally or include in the contract terms something like: "You could fly this right now" or "the wings will hold up to X amount of G-forces." There is a fine line between express warranties and "puffing." "Puffing" is basically hyping up the product. She can do that without incurring liability. An example of that would be "Mostly complete!", or "this is a great deal!"
Basically tell the truth and don't lie, and she should be good. (Just remember to get the contract in writing and include the language "as is" in the contract)
As to negligence liability: she needs to act as the reasonable and prudent person would act. So basically, don't lie, cheat, hide things that the buyer should know about.
But am with the above, I would get in touch with the AOPA legal services. It is worth the 25-50 bucks, especially for a $15,000-$100,000 item. The above is just some general info. AOPA will probably have a pre drafted contract along with advice.
