1) Yes. There has to be someone to sell. The buyer uses their credit card to buy the gear from you. They have gained an advantage over every one else not in the same position as you. P2W.
2) Not the same chance of a drop. The chance is probability it won't drop multiplied by the chance it won't drop for each individual player that an item can drop for. I e. .8 ^n where n is the number of players it can drop for. As n increases prob it won't drop decreases therefore prob it will drop somewhere is increased.
3) For you. If we just take away the RMT part of selling boosts and pretend it doesn't happen, the amount of gold a player like you can make is huge. This argument really has no bearing on P2W.
4) It is prestigious. Believe it or not some people will pay a lot of gold (or money) for the right vanity item. Mounts are included.