I spoke with my LBS about this, they won't do it as they aren't able to offer a guarantee on used bikes, and they'd rather sell a new one!
Gumtree, eBay or classifieds on here - eBay of course fleece you for ~10% (or more once you add paypal into the equation) but you will have the widest audienc. Given the number of stolen bikes out there, make sure you have proof of purchase when you do come to sell
Pretty much as Vicksters, my LBS (i asked about this some years ago) explained...they can't or won't offer a warranty, they have no history of the bike so don't know how its been treated, it competes against new bike sales which they'll make much more money out of...its a no brainer for them.
Edited to say, they also have to offer such a low price to get any profit on resale, they risk offending the customer, then risk losing a new sale into the bargain.