1. I wouldn't do it myself, nor would I sell clears. However my opinion..

If people pay others to clear content FOR them, then I don't really understand why they even bother playing a game. Just like my take on jump potions (not against them, but don't understand why you would).. If you pay for a game, and pay monthly for a subscription to said game, wouldn't you want to "play" said game, and not just pay other people to play it for you?

It's like buying a new game, starting it, then wanting to jump to the final boss straight away.

2. There are three reasons that come to mind -
a) The "I want it NOW" mentality.
b) Genuine players who don't actually want to pay for a carry, but want to pay for being in a party with other "competent" players (or players that know the fight) in order to gaurentee a clear/farm/whatever.
c) Players who cannot be bothered to form a learning party and learn the content but want the rewards.