My only advice when buying used is to ignore the price of the hardware when buying. It's all about credits. You need the seller to PROVE how many credits are left, if they don't know how to do it, you probably don't want to buy it from them
If someone has 0 credits left and wants to charge 250, it's not worth it, if you think you'll ever have another GM vehicle in the future. Cuz you'll need 2 for your car, and at least two for the next car, then you are almost at the price of buying new.
So I'd pay about 50-100 for the used hardware, then pay for credits, the only savings is if you won't buy credits in the future ever again, or don't wanna pay up front for credits and would rather buy them when you need them.
Also consider going with HPT Pro, then you don't have to take your computer with you to log, but then you can't see the logs until you get home. (also allows external sensors, like a proper boost gauge, turbo speed sensor, or EGT monitors for example)