Take a nice photo of the coin and give it a nice description. Photo is thebest advertisement there is.
I see shameless plugs for things selling on eBay on this site all the time in the WTS forum. Just put a link of whatever it is you're selling and talk about it.
Assuming you mean coins, I'd just go to the major coin forums. But, as others have said, good images and happy customers are the best advertising of all. Edited to agree with @jtlee321's post following mine... definitely use as many (accurate!) keywords in the title as you can.
Describe the coin as much as you can in the title. The more key words picked up in the buyers search the better. eBay really does not need to be advertised. But if you want traffic to your auction, then really nice photos and a great description will help you tremendously.
Some of the suggestions already mentioned are good ones. Using social media to promote can also be helpful if you take the time to do it. You can also look to see what the more successful people do and try to copy them. If you come up with anything good let me know please