Reasonable question, and the answer is that you cant. What you can do is gift the items bought with diamonds to someone else. You get the rubies, they get the item and they should send you gold.
It is generally customary to send gold first but as a new player, you would have hard time with that. People tend to trust higher level toons in more established guilds rather than new toons in new guilds. You may have to settle for sending item first. Select your customers by the same rule. If it is someone who is level 60+ and in respected guild, it is probably safe to send. If not... be careful.
-- Rustyx --- 92R / 92S / 92M on Reni (Guild KnightShift). Yes, running the new FOTM R/M, cause I am not elf enough to be WD/S.
Oh, and people who have more than 3 classes are clinically insane.