A lot depends on the plate. If it is specific to a certain type of car Eg. BMW or 911 etc, then a forum relating to those cars might be appropriate. I have a couple for sale on Reg Transfers, but have not had any success so far. It doesn't cost anything to list them there, but they take a percentage of the price. You can list it elsewhere at the same time, so eBay could be another place to list it.