I saw this on Imgur:
"Are all of these sellers flippers?"
Right now the top comment is "That's what a flipper would say!"
Everyone else claims that it is perfectly normal to remove the license plates and make it illegal to test drive the car and take it to a mechanic before buying.
According to the DMV (for states excluding the 12.15% that live in California), the seller is supposed to keep the plate. The buyer needs to drive directly to the DMV, do not pass Go, do not collect $200.