Mandolin Cafe is probably the best bet. Although it a mainly US site it does have many users on this side of the Atlantic.

Mandolin YK is a UK based site with a classified section but it does tend to be very quiet at the best of times.

On both sites you have to be a member to sell in the classifieds and I think on Mandolin Cafe you have to have posted in the fora a certain number of times.

Other than this, I think Trevor at TAMCO in Brighton will sell on consignment.