Besides the draft class of the most recent draft and Wood, Byrd, Moorman, possibly Levitre/McGee/McKelvin, I think anyone is expendable. I'd like to add Kyle Williams but he doesn't fit the mold for the new D scheme. The Bills have clearly hit rock bottom (but I am afraid to admit they can still sink some more) and have to/are rebuilding. I think Nix and gailey both know this and are just trying to put out the most competitive team they can with the players and talent they have. With a possible lockout looming, unfortunately it's not good business for the Bills to go out and spend tons of money on proven talent via free agency (which they didn't do this past offseason-as we all know). I think they are trying to evaluate and develop whatever talent they have on the roster. Going into this season I thought Edwards was realistically the only possible franchise QB the Bills had. Gailey and Nix needed to know if Trent could turn it around this year and do better. now that we know he can't be a franchise QB, it's time to move on. As much as we'd all like a proven veteran coming in here to salvage anything, it's highly unlikely. The bills should now start Fitz to try to spark the Offense and make the Bills competitive.
Trades will help Stockpile draft picks for next year which will hopefully be used to fill needs. This process is crucial to rebuilding. I don't know why that was not accomplished this past draft, of course unless this is all part of the plan. The Bills don't have many pieces to build a solid team around and this with this past draft it is too early to tell. So in my opinion, almost everyone is expendable.