One way to be totally legal is you can ask the seller to drive it to your location on their plates (assuming they have it registered/insured) or allow you to drive it on their plates and complete the transaction at your place. However, I would never do this due to the liability if you let them drive it on your plates or if I (the seller) drive it on my plates I run the risk of getting into an accident after I have the car sold and then what's the chances the buyer is going to complete that transaction?

Another way was I sold a car once and the buyer wanted to drive it home on my plates and I told him sorry, no. He went and got his brother-in-law who owned a garage to come back with him and put Repair plates on it for the drive home. Brother-in-law was really annoyed with me for not letting the buyer drive on my plates. Brother-in-law also told buyer he (BIL) needed to drive the vehicle on those plates. Also turned into a pain as the BIL decided he needed to now inspect every detail of the car. I finally told the buyer to take it or leave it but do it now!