does anyone know how to either make a listing for diamonds or how to trade them to a certain user?

For making a listing there's two little sections at the top of your Currency exchange,
on the left hand side above the listings, is the create an offer section - http://prntscr.com/7in4c9 (Keep in mind that the amount you're selling for is the TOTAL cost, not the cost per gem/gold. in this example I'm selling each diamond for 2850 gold so the total cost of 100 diamonds is 285,000 gold)
and on the right hand side is the search other peoples listings - http://prntscr.com/7in4it
Which is mostly self explanatory.

Edit: emphasis was placed. |D