There are basically 4 ways of selling gold:
1st: Sell to private buyers.
pro: Best Price
low chance of getting banned
con: private buyers are not easy to find for some sellers.
first sales may result without payment. You don't always know to whom you sell. Private buyers are not always trustworthy.
How to find private buyers? Ingame, WoW boards, Youtube,...
2nd: Sell on auction sites
pro: Good price
in the end you sell to private persons, and ban chances are not very high
con:
sometimes you have to wait ages until your gold is sold. Makes it useless, if you generate some 100k/day.
How to find gold auction sites? Google. Keywords: Buy Sell WoW Gold
3rd: Goldselling Websites
pro: You can be sure to get your money (if you do your homework fist, and dont sell to a scammer site)
you can be sure, that they almost always buy your gold.
con: low price
sometimes takes a while
pretty high ban chance
How to find gold websites? Google. Keywords: Buy Sell WoW Gold
4th: In fact it is same as 3rd, except with 1 more middleman. Find a trusted Buyer on ownedcore or similar sites
pro: You can sell your gold almost 24/7
the deal will be done in under 10 minutes most of the time
you can communicate with the seller directly via skype
con: high ban chance
low price
How to find trusted middleman? check out ownedcore or similar sites.