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.