The below links are helpful to the online seller. Feel free to leave a comment and let me know of any other good resources that you use. Or that you think would be helpful to other members.
Learn to Sell on eBay: Seller Central Best Practices
eBay Software for Sellers
Learn to Sell on eBay Advanced Selling Guide
eBay Education New to eBay?
Watch that Auction Fever
Add Auction Software
Charity Auction Software
Free Auction Ad Creator
Yahoo! Auction Manager
Internet Auctions: A Guide for Buyers & Sellers