Why sell? If from what I have read, HD-DVD players do a fine job of upconverting and just how long will those HD-DVD discs last, there should be a place somewhere in your house for the HD-DVD player. The only reason to sell is if you really want to respend that money on BD and you somehow believe that the BD copy of the movie in HD-DVD will be better -- which up to now on dual format studio releases has not been the case. Both seem to be pretty much equal.
I guess if you have recently purchased HD-DVD and do not have alot invested that would be a wise move but if you have already purchased 30 to 40 movies or more -- why sell?