How to report RTM?

I got clear evidence someone is RTM'ing.

How do I go about reporting this?

Exactly as Support have said, email them with as much proof as you have (screen shots etc)

Also do not expect immediate action, such RMT'rs can be used to ensnare those that trade them as well after a timed investigation allowing them to do their thing and letting GGG see who is doing what...this can lead to guilds dedicated to RMT getting banned and their collective inventories being wiped.