I collect license plates,too. That was an item that I have collected for years, and turn them into sculptures. But then about 5 years ago I got bit by BA (bicycle addiction) so Ive not brought home as many licence plates as I used to bring home, and am not building as many sculptures as I was then, now its bikes I like to build. Nice thing about collecting license plates is they dont take up as much space as bicycles, cant imagine owning 5000 bikes.
As far as pricing and selling license plates, EBAY completed auctions, gives you an idea what they are worth to the world. I used to buy alot of plates off of Ebay. Of course you could always put them for sale on Ratrodbike, Im sure there are collectors here, too. You can sell them at swap meets and flea markets, go where the car guys are at. Question is how much do you want to spend to sell them? And remember the age, condition, and state/county/country of the plates will greatly determine the value. Good Luck.