The Wine Spectator's Peter Meltzer addresses this topic quite well in a recent post in his Collecting Q&A online column (read it before continuing -- it's brief).

While auction houses tell you that you're buying "as is" and one could argue that the detailed disclosure of every fill level, label tear and other bottle condition is meant for you to factor this information (and any risk it may imply) into your bid amount, Peter's right that auction houses are empowered to make exceptions to preserve buyer relationships.  A lesser known fact is that their contractual agreement with the seller enables them to pass any liability incurred from accepting such a return back to the seller.

Every major auction house sales contract that I've seen has a "rescission of sale" clause.  This enables the auction house, usually in its "sole judgment", to accept a customer return for a variety of issues (including quality) and rescind the sale (meaning the seller doesn't get paid or reimburses the auction house if he's already been paid).  Whether the auction house actually bothers an important seller client for what may be a small adjustment is another matter but they have the recourse to do so.

On a related topic, you might enjoy reading an earlier post from 2006 called "Returning faulty wine to retailers."

P.S.    The photo is of a 1976 La Tache that you would not want to have received....