Filters
The first time I did an Ebay search on the term "Filters" the first page of results had auctions for:
That was on just the first page and their system reported there were more than 30,000 auctions for filters of one kind or another.
If you bu y any kind of filters I think you owe it to yourself to check them out on Ebay. I find that sellers who have been selling on Ebay for a while and who have high feedback write item descriptions that are extremely helpful. If you aren't quite sure what you are looking for you can give yourself quite an education by reading the item descriptions for filters that sound like what you are looking for.
Ebay is an extremely price sensitive marketplace and I find the guys selling slightly more expensive filters do a good job of describing exactly why their products are actually better than some which are cheaper. You may or may not believe their logic, but it certainly helps figure out what factors you should be considering.
Below is a list of upcoming filter auctions on Ebay. As I have time I will add some pages that break the results into specific types of filters so that finding what you need is easier.