Farming definitely works. I've heard rumors that some bosses give better drops the very first time but I think it's BS; several people have suggested X boss always drops Y legendary the first time, but I've never found this to be true (however I've had Boom and Bewm drop the Bonus Package twice on first runs through different playthroughs).
Particularly with Terramorphous and Bunker I've noticed there seems to be an explicitly set number of green or better, blue or better and purple or better drops. Legendaries are however much rarer than they were in Borderlands 2, so if you're basing your expectations off of BL1, especially Crawmerax's drops, you'll be disapointed. Legendaries drop at their own rate and most are exceedingly rare. I've farmed Warrior maybe 20 times or more and only seen the Conference Call and Impaler once for example.
Sell value is pretty irrelevant in terms of quality. Certain things like rocket shooting Assault Rifles have extremely high prices but aren't particularly better. A white rocket AR sells for more than a purple anything else, generally speaking.