Hi there, Zack from Online DMV here, hope I can lend a hand!
When you're selling a car, you need to make sure that you sign over the title to the buyer and provide them with a complete bill of sale. I'm not sure which state you're in, but some states require that you submit a special notice of sale form to the DMV (you can usually download this form and print it out from the DMV's website).
Otherwise, the buyer will then need to take that signed title and bill of sale to the local state DMV branch office to complete the transfer and then pay any fees for the transfer. The specific amount will vary from state to state. Either way, I would 100% suggest calling up your local state DMV to double-check the entire process.
I hope that helps, and if not, do let me know and I'll see what I can do to help!