With my SCT Xcal... they give you five times you can use it on different vehicles. You can go back and forth between stock and custom tunes on your vehicle... but if I were to sell mine, putting it back to stock, the next person to install it on their vehicle would only have four unlocks left. You can tell on the unit the number of times the unit has left. So if you can have the person you are buying it from connect it to his car, then you can access the information area and see. It might tell you if the unit has been on more than one vehicle. If it still has five unlocks, then he was the first owner. Once a unit is locked, to get it unlocked you have to contact SCT and pay .. .and pay.. and pay.. .LOL.
Make your decision wisely. Remember, if the unit has stock tunes, they were set for HIS car and might not be the best for yours. Which would mean you would have to go and purchase tunes from Bama or some other place that were customized for your vehicle. If both cars are basically stock, then you are good to go as the tunes should work fine for you. However, like I said, if he has say a CAI and you do not... you will most likely have a problem with the tune.
The old saying... Caveat Emptor... let the buyer be ware. It is always easier to part with the cash than it is to get a refund... LOL
Hope it all works out for you.
After owning a SCT.. I think I am going to look into getting a Diablo.. I am not crazy about their support, their stock tunes or their data logging features. Just my opinion... | 
