There's been a few topics about this recently. I think what's come out of them is:
1. Flames of Balthazar are the most gold per coin, but are hard to sell as not many people buy them consistently.
2. Elite Rit Tomes are next most profitable, and are a bit easier to sell in Kamadan.
3. Heavy Bags and Zaishen Keys also carry a pretty good amount of worth and Z-Keys are usually an easy sell in GToB.
Of course, if there's anything you need, get that first

.
P.S. Seconded on the 5.5k per Z-Key. They're a very easy sell at that price, a little tougher at 6k.