Here's what I know about eBay, their are some other very knowledgeable sellers that have posted some very valid tips.
1) Always use 12 photos - No matter what - It's going to give you the biggest bump. Make up photos, whatever it takes, show the item next to a ruler, the box it ships, anything you can think of.
2) If you have say 50 shirts to sell, always do GOOD TO CANCEL, this will maintain all your watchers and sold counts and this will also boost you in the search relevancy.
3) Being a Top-Rated Seller helps boost search rankings.. Their are pro's and con's some cons are you have to include tracking numbers, so no sending out baseball cards using postage stamps. It's going to count against you, you have to maintain 90% of what you sell with tracking. So some items will cost a little more to sell but the benefit is great.
a) Using fake tracking numbers to keep you in compliance will ultimately get you busted.
4) Always use Fast and Free shipping (They get a bump also in rankings). So when you're selling multiple "Fast and Free is the way to go" along with Good To Cancel, with 12 photos.
5) Ending times on auction, It's different based on gender and merchandise categories. Call eBay and ask for advice on when to end an auction for particular categories.
6) Theirs a new feature, not all accounts have it enable, its still in beta called, Relist your items up to 3 times automatically if it don't sell. Using this feature will re-list the items at no charge and no use of a free insertion.
7) If you need additional keywords to trigger that don't fit in a title, create Item Specifics for those keywords.
Always use the Ebay Global Shipping program, all you have to do is mail the item to ebays shipping center which is a commercial address, I sell a lot of items through it.
9) 1 day handling and 14 day+ return policy, gets you better search results.
10) Get yourself a label printer, if your doing first class postage on small packages, I use the QL-710W brother - Prints out a small label that can easily fit on small envelopes and packages.
11) Get yourself two scales, I have 3, one for measuring lbs, one for ounces and one for grams (I use for selling precious metals online)
12) Get yourself a box sizer (reducing box sizes save on packing material and dimensional weight).
13) If you can manage 2 ebay accounts, do it, once for being a top rated seller and one for selling things that don't qualify for top rated seller.
My dad is one of those crazy sellers on eBay with a gazillion feedback, I'm just starting out. I had many conversations with eBay regarding selling, this is what I have learned.