Do what I do -- carry the plates in the vehicle (as well as a screwdriver). My story if/when I get pulled over, "Sorry officer, I'm selling my car and was taking pictures of it, but forgot to reinstall the plate(s) -- I've got them right here and I can do it right now, how's that?". Even if he writes you a "fix-it" ticket, it'll only cost you a few bucks, if any (here in California you used to be able to get your fix-it ticket signed off by any officer with no fine, now you have to get it signed AND pay a fine ). However, I'd much rather pay a fix-it fine than a speeding ticket any day.

Hope this helps.

Brian