Nick,
These days with Ebay so popular, the music stores really do not trade in any where near as much as they used to.

Most of the majors have web sites/addresses so I would suggest you compose an email to send to all of them saying you are looking for a second-hand "whatever" so that the salesmen will know that when they get customers into the shop.

Also give them an indication of the price you are prepared to pay as they can then negotiate (probably) better trade-ins as they know they have a sale of the traded in gear.

Dennis