simple solution is to get rid of the exchange, not seen any other way to exchange credits ingame, cant exchange items
if ingame currency cannot be moved around, bye bye goldfarmers
to be honest this is perhaps the most immune game i have come accross as to get the best items, you need to explore or do PvP, the energy crdits only go towards basic items for your ship that you will replaces as soon as better loot is dropped...oh and 2nd or 3rd ships for your rank and your 1st is free...
The simplest solution Cryptic can employ, in my opinion, is a system where every mail, in-game or on the forum, that we get from an unsolicited source (for example from someone who's not on our friend/fleet lists) gets an expiration stamp put on it and is sent to a special inbox.
Once in the game, or on the forums, the player is alerted to the presence of unsolicited-mail and is able to either mark the sender as a spammer (escalating the incident through proper channels), or simply delete the message if its contents is actually innocent or was sent by mistake.
If however an unsolicited message expires without the player having deleted it or marked the sender as a spammer, the message is re-directed to a Community Moderator (or GM) to deal with it as they see fit.
Like some of you have mentioned, there is no solution to cut down Gold Sellers, not unless you disable in-game trades by making every piece of l00t "bound" and sell-able only to NPC's and also remove "gold" from the game. However, by allowing the players to block/monitor their mail inbox and providing them with easy tools to point out the Gold Sale spammers, the situation can be improved.