The fact is that the DMV can flag a deal for more tax at the time the vehicle is being registered. They wait several months before demanding more tax because they figure people won't be able to provide as much documentation because it gets thrown out or you can't find the seller. This happened to me twice in the last few years. Both were State surplus auction vehicles that were a pretty good deal. Luckily I had documentation so I sent it in and the tax collector went away.