If you sold a vehicle, you should have taken the license plates off of it right then.
However, in Nebraska, I sold a vehicle to someone, took the license plates off, but the other party didn't change over the title to their name.
I was sent the renewal tag notice. In my mind, it seemed to me it would carry more weight if I were to talk to them in person. So, when I had to renew my tags on my new car, I took the post card in for the old car.
I explained what happened, showed them my bill of sale. It was pretty funny. They said, "OK" and tore up the post card and threw it away. They didn't notate it on the computer.
When the other dude got pulled over for expired tags, and he did, he tried to bluster his way through. They ran the plates and it showed that "I" was still the owner. So he had to admit that he bought the car from me but didn't change anything over. He would also be cited for not having insurance. It would have been cheaper for him just to get everything changed over instead of waiting to get pulled over.
I still had the paperwork for the old car showing the sale, in case the cops were to call me, I could prove that I sold the car and what date I sold it.
If you are concerned, call your DMV and ask about this. Generally speaking, most are pretty helpful. Don't call on Monday morning. Don't call on the first or last day of the month. Don't call around lunch. I've found Tuesday or Wednesday mornings work best. Good luck.