Sure when you're selling one or two items a week on eBay then you probably don't need any extra resources or tools. Just the main eBay site but if you're listing and revising hundreds of items a week then you will definitely need some extra tools beyond the main eBay website. Here are some of the best:
1. Turbo Lister 2.0
This stand alone program allows you to list, duplicate and revise many items at once. It also eliminates the need to schedule listings because you can save up all your listings and upload them all at once. It is not without it's problems however it is very buggy at times, documentation is scarce and it encounters strange errors while trying to upload. So why do I recommend it? Because it is the only free listing tool I found to use with Ebay. You can find Turbo Lister 2.0 here.
2. Outright Accounting Application
Outright is an online accounting application that allows you to easily track your eBay income. It imports all your PayPal payments and deducts all eBay and PayPal fees. Upon download it will import the last three months of payments and fees. To go back further you have to enter each transaction manually. Once you start using this program tracking profits becomes very easy. You just have to enter shipping costs and other expenses manually all eBay fees and payments are handled automatically and it is free.
3. AdShip Ad Supported Shipping
Shipping supplies, printer cartridges, paper all these things are EBay expenses and while we may not think about the cost much it can add up. One way to offset some these cost is with ad supported shipping by AdShip. When an order is placed and postage is paid (Using PayPal Shipping) then you log onto the AdShip program and choose an Ad to be included in an email confirmation to the buyer. For allowing these Ads you can be reimbursed up to $0.25 per package. That's enough to cover bubble mailers and printer costs. One drawback is it only works with domestic orders and each Ad must be chosen one at a time instead of in bulk. Otherwise it's great. Check it out here.
Update: Adship has added the ability to print and choose ads in bulk.
4. PayPal Multi-order Shipping
This simple tool allows you to bulk print all domestic Ebay orders that have been placed. It automatically imports all new orders at start up. It does have some problems though. For one it only supports domestic shipments not international. If you have a normal printer it will use one page per shipment although it could probably print much smaller. Anyway those are small complaints and it's free.
Update: While Ebay has added the ability to print postage directly from the the Selling Manager application I still find Multi-Order to be faster when doing bulk orders.
5. Ebay Selling Manager
Instead of having Ebay's default My Ebay page try subscribing to Ebay's Selling Manager. You will then be able to leave feedback in bulk for buyer. Handle non-paying bidder cases in bulk. Relist in bulk and see more detailed sales information. It's free and you can always switch to the old view at anytime.