Never buy a used LiPo, I consider them disposable after 6 months old. If I include them in a used car deal, I don't give them any value and just include as a way to sweeten the deal.

Most of the hard cases on my budget brand batteries tend to swell after 6 months to a year of use... however if not properly cared you can cause them to swell on the very first use. Very easy to do if you charge them in a cool A/C house, then take them outside in extreme heat where heat causes voltage to increase. There was a guy on URC (TheJANG) who charged his batteries at home, placed the batteries in the trunk of his car, then went on a 2 hour drive to the beach and when he got to the beach all of his batteries were swollen and completely ruined, all brand new packs too!

If the guy shipped your car in the middle of summer and the delivery guy left your box inside a hot delivery truck for hours on end, would not be surprised if that damaged the pack too. I try to have my packs shipped in cooler months when possible.