Found this on reddit, thought you guys would like.
Somewhat server dependant. A lot of people say that gems sell for more or enchanting mats sell for more. On my low populated pve server this is the best way haha. Just double check your auction house first.
3990 Honor -> 600 gold. Most gold-effecient way to spend your honor (at least on my server). - Imgur
Apparently the brutal gladiators shiv vendors for 15g and costs 105 honor. So 604g if you're honor capped. Which is a lot better than anything else, at least on my server so, yeah.
Only downside is you have to wait the 2 hours or w/e for it to vendor back for gold and not honor.
Edit: Hanky found a better item.