There is an FAA issue here but I will leave that to the experts. With respect to contract law, assuming this was an MD product one would look at the venue section of the contract. If the venue is Arizona Law, then...
If the used parts were fully disclosed, and the buyer accepted it, there would be no issue. If they were not disclosed, the buyer may have an issue if they could show 1) that these used parts were clearly used and 2) that there were any damages associated with putting on these parts.
Take a blower bearing unit and belt. Look a the life and pro-rate. The damages end up being a few thousand dollars. Who wants to spend 100k litigating over those damages.
The moral of the story is two fold:
1) Do not take delivery of any item prior to an appropriate inspection. In cases such as mentioned on the thread, they would be disclosed on the component cards and so you would have notice. Work it out before you pay the balance owed.
2) The most important part. Deposits on aircraft sent to the manufacturer are unsecured debts. You send MD or Sikorsky or anyone a deposit and the manufacturer runs out of money or gets into trouble, your deposit is at serious risk. If the manufacturer is having problems, escrow your deposit so that it is not the property of the manufacturer but is held in a neutral account.