Playerauctions.com for selling direct to customers gives $5-$6 per 1000 gold at the moment, while company websites usually pay no more than around $3 per 1000 gold.
Personally, however, I've always sold to company websites. Since usually the account gets banned for gold trading within a few hours to a few days afterward,* I need to instantly sell several tens of thousands of gold on multiple realms, within a hour or two, something satisfied by the Chinese company websites but not by waiting for customers.
* (I know that's the cause of the ban for certain, as I make my gold by just legitimate methods up to the point of the sale and have accounts for weeks without problem, before being banned immediately after gold sale).
Sales to customers directly might work if one could avoid a ban, but there's no info anywhere on these forums or anywhere else I've seen on how to reliably avoid such, a question nobody ever even seems to ask or study. (Such is weird; I know many people here just get their gold by scamming and thus don't care if the account gets banned after sale, but surely some people acquire it by other methods like my auction house trading and would care?).
So it'd be pretty risky if you had a lot to sell and got banned after selling the first small fraction while waiting for more customers. If you had enough money and time, you might be able to yourself determine what results in bans by trial and error.
I know myself, in my experience:
- Selling by guild bank deposit to company = always banned afterward
- Selling by face-to-face trade to company = always banned afterward, except one time when I sold only a mere 2000g on a single realm and never was banned
- Selling to some regular U.S.-IP-address individual player = unknown, not tried yet