A good way I’ve found to gather gold, is to find an area with red war gold humanoids that are equal or a little below your level, and go on a killing spree. Collect everything they drop.

Once your inventory is full, sell the grey items, place the good things like cloth and green or better items with nice stats in the auction house, and resume killing. Doing that for a few hours should net you a good amount of gold. Element of importance here is your talent spec.

Affliction and Demo specs are quite well suited towards farming while Destro has a hard time to keep going without downtime. Enemies that yield good drops, plus some generic info, it is worth noting that some grey items sell for a very low amount, while others provide a good deal of money.

For level 50+, try some of the following. The plague bats in Eastern Plague lands also drop some very well selling stuff, such as Bat ears (8s each), Large bat fangs (16s each) and Sleek Bat Pelts (12s each).

They also drop Evil Bat Eyes, which are used as a turning for Dark moon Faire Cards. Hearts of Fire sell nicely; the best place to get them is off of Entropic Beasts (51-52) in Felwood, north of Blood venom Falls.