I went and finished getting every class to 80 a couple of months ago. I had my fun, but I wanted to see if I can sell any of them now and just keep the three I prefer playing the most (Paladin, Druid, and Mage) and at least get something out of it. Unfortunately, it seems you can't add character descriptions anymore and you can't lower the price. Said characters have been on the Marketplace for a little over a day so far?
Is there anything I should know that will make my characters sell easier?
If you are talking about "fresh" characters (ie no interesting titles, achievements, mounts...), they generally only sell when they only have bis pieces.
Little over a day isn't long time. It's still a 30 coin transaction, and that isn't exactly cheap. It may take weeks, depending on equipment/achievements/proffs/mounts. Or some other factors like class, supply and demand of the class in raids and in trade section.
I had priest with one or two BoE's, proffs just enough to get the perks, probably fast flying. Took few postings before I got a sale (3 weeks or a month).
At the same time, I was selling a Warlock too. Many good BoE's (chest, wrists, legs, boots) and maybe some extras. Proffs, fast flying. Took longer to sell than the priest (roughly 1 and half months).