After you sent egold, or received, you can rate tbm.
(the egold must be 26 or more than 26 to rate a tbm)
Go to "The Black Market"
You will see all the sent,received egold.
Pick the trade.
Scroll a bit, you'll see "Rate this Transaction"
The text or whatever(Even if you won't write anything,It's okay)
Check this thread.