Before I ever list an item on eBay, I search to see what items like mine have sold in the past few weeks. This step also helps me determine the best possible category for my item, particularly if its something I do not know a whole lot about. With so many different categories on eBay it can be very easy to incorrectly categorize your item, leading to no traffic and subsequently no bids.
Once you find an item that is similar to what you are selling, you will want to view the listing and click the link that states "Sell One Like This". This link will open the item listing template and will pre-populate it with the same category as the item you just found.
If you happen to find a similar item in 2 different categories, eBay allows you to list in both, for a small fee of course. I have done this quite frequently when with my craft stuff as it helps increase the amount of traffic my item receives.