Now your confusing RMT with multiboxing, not all multiboxers are RMT and not all RMT multibox.
Also you seem confused about the impact RMT has on the economy, they don't sell items for ungodly prices, they don't want to wait for items to sell and risk more time where they can be caught and have items removed and characters banned. If anything they drive prices way down making them more affordable for all and killing profit margins for legitimate players.