Hate to say it, but I think there are the some ideas from other games that would work fairly swimmingly if SE actually wants to get rid of Gil Sellers.

EQ2 has a setup where it filters Tells that constitute as spam, I dunno the full elaboration of this feature, but in conjunction with the Mail and Broker restrictions on free accounts they have literally abolished Plat Sellers.

Rift also has mysteriously lost a significant amount of their currency sellers; owing partly to the implementation of REX as well as a severe crackdown on...Mailing and Broker services!

You get where I'm going with this, correct?

Two things can be done to prevent it far more readily since Bans/Deletions don't seem to be doing much at all. We need to make it significantly more costly for RMT to operate on our servers.

--Require finishing the Level 20 Main Scenario quest to use the Marketboard, Trade only allowed with Friends or Free Company members until that point as well.
--Accounts flagged via player report for tell spam or suspicious activity (I.E. these weird 99 stacks of Tin Ores/Bat Wings for 1 gil apiece on broker) will have Mail, Marketboard, and /tell functions disabled pending an investigation.

This would more effectively cripple the RMT because it would make the players more obvious. The second one might require some player policing, but it is still more useful than how we're going about it now -- people can't report fast enough to counteract the remake-and-spam again tactics of the RMTers. Just an idea though.