Thank you for all the comments. I think if we both had a better understanding, this deal would have not gone bad and I would have had a new S4 and the buyer would have had the NSX that he wanted. Many of the posts have questions that I would like to answer here.
1. The sales tax law in Pennsylvania: The Pennsylvania buyer pays 6% sales tax on the amount of cash that changes hands whenever he buys a car, either privatly or through a dealer. For example: If I buy a $50,000 car outright I pay $3,000 of sales tax. If I buy the car, but trade $20,000 worth of good (diamonds, guns, merchandise, etc), I pay sales tax on the remaining $30,000 ($1800). If I trade a car of equal or greater value, then there is no sales tax to pay because I'm not outlaying any cash.
2. What is a courtesy trade. It is an arrangement made with an authorized car dealer that they will trade your car and immediatly resell it for the beforehand agreed upon amount to the buyer. Some dealers will do this; others will not because it costs them effort and the only benifit to them is in selling you a car that you otherwise wouldn't buy if they would not extend you the courtesy.
3. Is it legal? Yes! It is entirely legal. Specifically, the dealer agrees to buy your car and resell it immediatly to the buyer. Now why would a dealer do such a thing as buy and sell a car without making a dime on it? Because they are doing it as a favor to you to get you to buy their car. Since the dealer does not touch the car, other than doing the paperwork, they have no expense in it, such as prep, advertisment or other expenses.
4. Is it safe for the buyer? In fact, it is safer than buying the car privatly. The buyer is actually buying the car from the car dealer, and if a problem is found they have recourse with the dealer, not the private buyer. This actually puts the dealer at a liability risk.
In my specific case, the Audi dealer has a policy that they will not do courtesy trades, but would have accomodated me because I have purchased three cars form them in the past. However, the y required me to have the car inspected against frame and prior accident damage at a cost to me of $200 because they are lible for the car, not me.
5. Why can't the dealer just keep the car and refuse to sell it to the buyer at the prearranged price? For a number of reasons: A: The arrangement is already setup prior to anything being signed. B: If the dealer would cheat and do this, the deal would be stopped on the spot. All the parties are present and signing the documents. For example, Lets say that happned: A: I would not buy the new car from them. B: They would need to believe they can sell the car at the profit margin they need. C: They believe they could get away with such dirty behavior. This just doesn't happen, especially with a respected new car dealer.
For an out of state buyer; how does the courtesy trade transaction differ from a private transaction:
Private: The buyer pays me. I sign the title over to him. We go to a a notory who handles car transfers. The title is notorized where I signed and the buyer can purchase a transit plate so he can drive the car home. Once home, he brings the title to where ever does car transfers and applies for a perminent plate. He also pays the applicable taxes. He gets a title issued from his state in the mail.
Courtesy trade: The buyer pays me or the dealer for the agreed upon amount, which may be less than the acutal amount. The title is signed by both me and the dealer. New title origionating in the state of the buyer is applied for. A transit plate is provided along with temporary registration. The buyer drives (or ships) the car home. A new title issued from his home state is mailed to him in 7 to 10 days. He then applies for perminant tags and pays applicable taxes. If things go wrong, his recourse is with the dealer, not me. And they say.. If you are going to sue someone, sue the entity with the most money. And that sure isn't me.