It just has to be uninstalled from the last car. When you flash your car with the AP it will read the VIN of the car and "marries" the AP to that car. The owner/seller should be able to tell you that. If he didnt unmarry it, the AP is (kind of) useless and the car he turned in is "locked" as far as the ECU goes. If it is married you can send it back to Cobb and they can/will unlock it for a price. As far as it working for your car, go to Cobbs website, look up the SUB number for your year and then the SUB number for the one you want to buy, if they are the same number, it will work. You will just have to upload (from Cobbs website) the correct maps for your MY car.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX