There are many private plate sellers around, best thing to do is to ask them for a quote on what the plate will fetch on the market.
Most of them will advertise the plate for free but will take the sale fee out of what the plate sells for.
If you advertise through more than one company, remember to inform the others if the plate is sold.
Or you could try Ebay.
Is the plate currently on a car? is it mot'd and taxed ?