Webber is probably second to Darius Miles in the list of most difficult players to trade.
Probably, although Kenyon Martin could give him a run for his money. Especially since C-Webb's contract is up in a couple years, teams vying for lottery or a big free agent pickup may want to deal for him. K-Mart, on the other hand, has a 5 year deal, ending in I believe a $16 million contract.
