1) Pretty much, even though some are more useful than others.
2) This really depends on the seller-buyer matching. Those who go crazy pulling packs probably don't even bother with selling them and just use them as feeders or for LP. I've also seen sets of five going for 1-2te. Personally, I price them at 0.5te each (allowing for goddesses), which nets me some buyers.
3) Some stand out, but I'm not aware if they are worth more. For every useful common 4*, there is a better LE 4*. If you're gonna pay with te, might as well go all out and pay for the LE ones.
Relevant thread: http://www.fantasicaunofficial.com/t9233-useful-4s-to-keep
Originally, I would price Zenobia higher at 1-2te since she was the only 4* with Iron Wall, but with the release of LE 4*s that also have Iron Wall, that price point will probably be lowered to 0.5te. She does have good arena damage though, and I'm not sure how the new Iron Wall units compare in that regard, so she might still be worth more than the other common 4*s.