Have a look at the Music Industries Association website, and see if you can find a music shop locally that has membership:
http://www.mia.org.uk/retailers/
The MIA is a trade organisation, but its members are some of the most reputable. If a local member shop can't help you, then it should be able to point you in the direction of someone who can. My local shop (also a member shop) often sells stuff on commission for people, and their terms are pretty good.
As a matter of interest, what type/make is it?