I've been selling cards on and off for the last couple of years now on ebay - and I still don't really know what will sell outside of my specialty. I sell a lot of Philippine postcards and I have a tight group of bidders (almost always the same group of people) bidding on the cards.
My suggestion would be to group your cards according to topics. Angel cards always attract some buyers. Railroad cards are great - if you have some of the rare depot/railroad stations and RPPC are always great. Children RPPC are great - esp. if they have something interesting in them (not just posing for a pic but has a toy, animal or something). Some areas are just too saturated - Clapsaddle's signed cards are a dime-a-dozen so only those rare cards get high bids. And Holloween cards sell big-time. Swastikas and confederacy flags also do well for me.
But still, it is hard to find out what really sells. What I do is to dip my feet slowly into an area, until I find what works and what doesn't. I try to list a limited number of cards on a particular topic (maybe 2-3) and after a week of running the auction I can get a sense of whether it's working or not. I try to study the completed items, and sometimes it helps. But sometimes the buyers are just there, and sometimes just not.
As for pricing, some categories like Philippines I start at $8.99-$15.99, and still get bids. But for US views postcards, if I start at $6.99, I get emails from onlookers saying my price is too high and they would bid if only I lowered the price. So now I start my US views at $4.99 as well as poorly looking cards. I have this one card that is in my "poor/damaged" card box and well, got scanned somehow because I was selling some Indiana train station cards last week. Never expected to get a single bid, much less 14 bids
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=20208&item=2282245016&rd=1
You never really know (and that's the fun part). But it is good to cultivate a particular topic and if it does well, focus on it.
Good luck
Isabel (gad88)