Dealers also have to offer at least a 3 month warranty, which will be at the expense of the dealer should anything go wrong. Also, the dealer has to sell the used item at a very attractive price, which is a lot lower than it was 10-15 years ago due to stupid bargains you can now get online. Add to that the VAT as record spot mentions and you'll more than likely not get quite what you expect. We'd always recommend selling privately as that way you'll get more for it than you would part ex'ing.
We have a second hand board in store for those wishing to sell their items privately, which is free for all to add to.