I'll speak a little bit about my personnal experience as run seller.
At first i didn't wanted to sell runs because i was thinking like some of you "yeah it's just encouraging players to buy gils from RMT" but one day a friend i made into the game told me "come on we need one more DPS for T9 sale" and since i was the only person online i accepted. After this experience i kept selling runs for one reason beside the money factor : it helps you to improve yourself. When you are doing a 7-man content, the room for mistakes is getting really tight (okay now with 15% echo on Scob and i130 it's not a challenge anymore, but trust me, T10-13 7-man are kinda challenging o.o). During a sale, some good players from different statics gather (depending on disponibilities) so you can actually compare yourself to them (without any kind of parser) just by looking at their rotation and things like that. And even then, i keep helping some players on PF for free when i see a PF like "help on Coil/Primal/whatever else, exp players welcome" and some players who are selling runs with me are also helping for free like that.From what i saw, run buyers are mostly big crafters, dominating the economy on the server, but sometimes it happens that a player tell us "ok wait i'll ask to my RMT to give me 15m". When this kind of thing happen, we just report him and refuse to sell the run. Ppl who are selling runs like me just for the taste of the challenge won't accept things like encouraging RMT, but well, unfortunately there are also sellers who just want to make money without any kind of mercy :/ you can't really change that...