You have a couple of options really, you can go through a number plate dealer (there are hundreds out there), or you can advertise your number plates yourself privately.
Going through a dealer can seem like a good idea, you don't have to advertise your plate yourself, and hopefully they will find a buyer for you, however, what they tend to do if you want to sell your plate with them, is offer "free valuation" beforehand, they then advertise your number plate for sale, on their website, at a much higher price than they offered you in your valuation!? Why? Because they keep a percentage of your number plates selling price as their 'commission' for advertising it.
The other option people tend to look at, as I mentioned, is eBay. The only thing is, their fees can end up being as much as going though a dealer, as they charge a listing fee, relisting fees if it doesn't sell, and a final value fee of roughly 10% if/when it does sell.
I had a couple of personalised/cherished number plates that I had bought over the years, and wanted to sell on again, but I found there was nowhere I could just advertise them without it costing me an arm and a leg in commission and fees, which is why I set up my website, as a place for people to advertise their numberplates privately, with no commission, relisting fees or final value fees. Have a look at it if you want, its....
www.Plate-Trader.com
Good luck with the sale!