There's also more than one SCT tuner out there, so you need to find out which one the guy is selling to see if it is really a deal or not. The most common tuners you'll come across are the X2, X3, and SF3. The only one I would even consider buying used is the SF3. The X2 and X3 cost nearly the same price to buy new as the SF3, but they have no pre-loaded tune files. IIRC, they are capable of holding up to 5 custom tunes, but any tunes that are already on them are not for your car so those tunes would be basically useless to you and would be taking up room in the device that could hold a tune you want for your car. The SF3 comes pre-loaded with several tunes for many different models, so you don't have to buy a custom tune to get some use out of it, plus it still holds some custom tunes if want to go that route.
Since the X3 costs almost as much, people who bought it and now want to sell it as used tend to think it's worth more than it really is given the availability and better value of the SF3. Just my $.02
As far as unlocking one, the last time I looked at the SCT website they charged $150 to unlock an SF3.
