The popular transmog sets can sell pretty good, depending on your server. I sold over 2k worth of vanilla greens today. Most selling for several 100 each.

What I normally do is camp the rare spawns in vanilla with my alts. I'm actually trying to farm the overlord plate set, but end up getting tons of other drops that I just throw on the AH. Burning steppes and tanaris both are pretty good. Also, in winter spring, up north there is a spot you can put an alt where 2 diff rare tigers spawn. Just log in your alts a couple times a day and kill the rares.

I was doing this on a low pop server, so there was always lots of rares up when I checked. Sometimes 6 or 7 in one zone. I'm now on a high pop server, so it will be interesting to see the difference.

And of course running old raids and dungeons for transmog gear works too. For me this is the only bad thing about transmog, is that I now feel compelled to loot everything.