A legit seller can remove their plates and provide you with a temp tag so you can get it home and then you're on the hook to get it registered, and they don't have to worry about getting a ticket or arrested for someone else being stupid with their old car. Since plates, color, make and model are pretty much all anyone can use to identify a vehicle, makes sense to remove one major link to it so you don't get dragged into dealing with all that crap, even if you can prove it was sold still imagine it would be a major headache. I get it, the seller technically owns those plates/tags, so better to just take them off and get the new owner to deal with it.
But it is a yellow flag situation - proceed with caution - since there is so many bad things that can be linked to temp tags.
Besides the obvious - bought from dealer/seller and need to get vehicle registered/insured so temp tags allow you to use vehicle until real tags come in... they use it here for a few reasons: to avoid having to get insurance/inspections and drive it until it dies or gets caught, or thieves (BIG deal around here) so they can't trace the VIN and can try to avoid being caught in their getaway car.
There are flippers/shady dealerships that are pretending to be a private seller, but actually buying vehicles in lots from the big auto auctions, and then don't bother getting real plates/registration and flip the car to some unsuspecting buyer. Car may be fine, but you have to take anything the "owner" claims as unlikely because they probably only have driven it a few times around the block to make sure it basically runs.
Temp tag selling is illegal, but happens all the time. Someone will offer to provide the tags if you tell them the make & model (but they make up the address/name) and pay them like $50 for what should cost under $10. It is illegal to do this in my state (sell state issued documents specifically calls out the temp tag stuff) but still so many people think it's fine since they are pretending they have no knowledge of why anyone would pay them more than triple the price for something anyone can do themselves if they are doing it legally... riiiiiight.
Read about one guy in the last year that actually bought a stolen vehicle somehow that had temp tags sold by a supposed private sale from a ad off of craigslist or offerup or something. The VIN number was faked in the obvious places like the dash/door, but they didn't match the real VIN in the motor area (so one thing to do to CYA is look up the prospective vehicle's VIN locations and make sure they all match and are clean!)
