Biscuit is a useful Firefox add-on which helps you delete unwanted cookies, while carefully preserving everything else.
Once installed, clicking Tools > Show Cookies displays a new dialog detailing your current cookies. And at its most basic, this works much like Firefox does normally: you can scroll the list, view particular cookies, then delete just those or the entire set.
You can also search the list for cookies you'd like to keep, though, such as your eBay login. Select and right-click these, click "Check", and you're done - clicking "Remove All Cookies" will delete unchecked cookies, but leave everything else untouched.
Sometimes you may actually want to delete all your cookies, of course, but the developer has thought of that. A "cancel cookie preservation temporarily" option makes it easy to wipe all your cookies on demand, while still preserving them in future.
Better still, there are a host of configuration options to help you get Biscuit working exactly as you'd like. You can choose to mark the cookies you'd like to preserve (opt-in), or delete (opt-out). Session cookies can be preserved; unwanted cookies can be removed when Firefox closes down, and you can even have Biscuit's menu item hidden, preventing others from tinkering with your settings.