In the last three years of serious selling I've learned a lot. I didn't know how much I didn't know in the beginning. My sources of knowledge in the beginning were mainly a few reseller blogs I followed (see some of those on the sidebar) and reading the discussion boards on ebay. Reseller blogs are a wealth of knowledge. From ideas on what sells to advice on the ins and outs of selling. Some people even do Youtube videos and podcasts helping other ebayers along. The discussion boards on ebay have their good points and bad. You tend to see a lot of ebay bashing on the boards. You can still get helpful advice there if you have a specific question because there are a lot of experts on the ebay boards.
My best source of knowledge has been a discussion board called Ebay Underground. It's not a really busy board but you could spend weeks just going through the old threads and learning very valuable stuff. If you don't belong to such a group you are missing out on a good source of information.
Ebay Underground can be found at http://ebug.proboards.com/. You do have to register in order to view the board.
I also highly recommend the podcasts on Ebay Scavengers. They are each about 20 minutes in length. They are informative and great to listen to as you are doing your shipping or picture taking. No registration required.
If you are a newish seller I highly recommend using blogs and discussion boards to gain knowledge and answer your questions.
Do you have any other good sources of information? If so leave a comment and tell me where you learn! I'm always on the lookout for more.