So I had a similar problem with my last Bike. I told the guy that he could use my plates whilst travelling down to his home to make life a bit easier (no cops pulling him etc). I also asked him to send me the plate once he arrived home and he promised he would.
Anyway a month later I still hadn't received my "personal plates" back! I pinged him and asked him what was going on. All he had to do was take them into his local DMV and they would send them back to me.
Anyway, I was getting a bit p!ssed off after a few more weeks so I sent him a letter that said that if he didn't send them I will be going into the DMV and letting them know that he was committing a federal violation by riding with my plates and the DMV sees this as a felony. Anyway, low and behold, my plates arrived 3 days later in the mail!
Learned my lesson. Take your personal plates off before selling a vehicle! BTW, I did get a signed letter of sale which we both signed and I sent in my release of the vehicle and all liability to the DMV. I think the guy was just too lazy and was having too much fun on his new motorcycle to do the paperwork! Can't balem him. It was a nice Motorcycle and he got a bargain!