Print out a instruction guide with a parts inventory for the exact kit being sold and go through the whole thing noting any parts that are missing. Procharger stuff gettable on superchargersonline but missing brackets will drive the price down substantially. Hardware is not a huge deal to be missing nor are messed up couplers or v-bands but again, the more stuff that is missing (if any) the more you haggle.
But yes the biggest thing is the headunit itself. Should be minimal or no shaft play side to side or in and out. If there is anything substantial then I'd still buy a complete kit but tell him it will be minus the price to get Procharger to go through it or at least a good chunk of it.
And at 1,000 miles I would hope he would have a receipt that is pretty recent unless its been sitting in his garage for years waiting to go on the car.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.