You cannot use your coins to get more coins, the old famous scheme, buy boe tabard lightbringer for 5 coins -- sell it for 80k gold -- sell 80k gold for 10 coins on trade -- repeat, (or whatever the exact price was) has been removed, due to obvious reasons.
Best thing for you to do is save the coins, farm gold ingame and sell for coins on trade, Don't forget you can also sell items for coins directly on trade, like 90str trinket from the darkmoon cards might go for quite a few coins for example.
Anyway there's no easy way anymore, people have been using every little way possible to make easy money that there's no ways left that hasent been exposed (well not as BIG as before anyway) shhhh.
You can still make quite a lot of gold from taking advantage of the market though, in items that sells for higher price then it should sometimes.
The old "easy money making" ways have made a lot people take control (or trying to) of the market since that's easiest way nowadays, one of the most famous being the "elementium plated mechano hog pipes" in k3 where a group of people set up an alliance to hoard the item, buying it for 1,5k and re selling it for 4-6-8K gold, using a macro faster then any other user could click to buy it, and having schedule every 2 hours and 30 mins (time for item to appear again in npc) to have next guy ready for next time to take it, by still trying to create the illusion of a fair market the pipe is sold by different names this was hidden quite some time from everyone, it still is.
There's a lot of thought put down in all different schemes people are up to but whatever you do, don't fall as low as trying to scam, like these guys selling 11111 gold for 30 coins (eleven thousand gold) right next to a guy selling100000 for 30 coins and since you sor the list by "ammount oif coins sold" and NOT by the ammount of "Gold gained" it ends up right next to or close to eachother, fooling quite a lot of players that's not used to it, wish warmane would just bann those ****ers, it's ovious scam and against game rules, and it's easier to miss then you think, Oviously I se it a mile away but not everyone does, and that shoulden't matter when it's still scam in the end.
Anyway everyone have their own way of making gold that they figured out brings most gold (some better then others) but they are secrets kept hidden in almost every case go out there and find your own, most of ous did.