The problem with Sanchez is this: You need poise and field general skills to succeed in the NFL. Those skills generally are not teachable. You generally hae them or you don't. Russell Wilson, for example, had demonstrated in college he had that skill. Luck did. That guy at Cincy did. Sanchez has demonstrated in his years in New York that he doesn't have it. Not to say that he won't improve or that he can't change, but the fact that he showed that he wasn't in charge of that offense means he's a low probability guy in terms of possible success elsewhere.
Couple that with the fact that, apparently, Gailey's offense is difficult to master and it makes Sanchez a guy you'd better pass on. The Bills need a guy who hasn't already failed someplace else. Losman and Edwards were better choices than Sanchez, because before they arrived in Buffalo they hadn't failed elsewhere.
Of starters in the league, Rivers is probably the only one who might make sense. He certainly has struggled lately, but at least he has had success in the league for an extended period. You can't have three years in the NFL with a passer rating over 100 without knowing how to get the job done. He probably has 5-7 years left on his career, and a change of scenery might do him good. He has a quick release, which is important in Gailey's offense.
I don't see another guy in the NFL who would be worth a major effort to get. In any case, not Sanchez.