PlayerAuctions is the safest site, 0% chance of scam if you follow their guidelines since they act as a MM for every trade.
Besides that, just buy/sell from/to any of the highly vouched members here at Sythe.
OT: As the first poster stated, Playerauctions is by far the best site to get gold from as most will deliver within 20 minutes of purchase and there is literally 0% chance of getting scammed. I have personally bought over 1B gp from magicgoldseller on there and have had 0 bans and 0 scams. All orders were also completed under 10 minutes.
So in short use Playerauctions as you won't be disappointed.
I remember the first time that I used Player Auction where I'm a complete noob regarding about how to use them. It's actually my fault wherein I dealt with a buyer through PM and didn't followed the instruction as stated which lead me to get scammed.
Anyway, I recommend that you should read all the instruction on the site before you intend on using it.