Don't really need a guide...
People usually sell gold for 25 EC a piece, or a little higher depending on the player.
Choose the item u want, for example a weapon skin...
Tell them which u want for the 30 gold..
The player gets it for u n in a trade window you give them the EC's for it.
30X25=750 ec done deal :3
Same with any item in the boutique ^.^