My LBS often has a used bike or so on the floor. A Rodgriguez the last time I was in. If he takes a cut I don't know how much. Perhaps it's as a favor to the owner who is using the sale to upgrade or thin the herd. Some of the bikes in his loaner fleet are trade ins from customers and so his build anyway.
His trade ins are all excellent bikes. While I waited for the Mondonico to be built I rode a bike from his "trade in/loaner fleet". The one closest to my size is a Della Santa. Who gets to just loan one of those? I believe it was a trade in and a delight to ride
but a little big, hence the Mondonico.
http://www.youtube.com/watch?v=MGAUVUukQak
I'd hate to sell any of mine but if I did would go through my LBS.