This is a known issue with CSS and eBay. I have been dealing with it as well. For me it seemed to occur when I revised the listing in eBay. I have a couple things you can try...use a different browser to push your listing, put the CSS at the bottom of your html instead of the top, and if you are revising be sure to click the "save draft" button.
You can find more info here...http://answercenter.ebay.com/search.jspa
Use CSS as the search term.
Good luck. If you come to any good conclusions be sure to let me know and if you want me to take a look at you code, give me a link to your listing.