While you can sell most things on eBay, quite a few things are banned. If you try to sell any of these things then eBay will remove your auction and all bids will be void.
Here is eBay's full list of prohibited or questionable items:
Airline and Transit Related Items
Alcohol (also see Wine)
Animals and Wildlife Products
Brand Name Misuse
Catalytic Converters and Test Pipes
Charity or Fundraising Listings
Compilation and Informational Media
Contracts and Tickets
Counterfeit Currency and Stamps
Drugs & Drug Paraphernalia
Embargoed Goods and Prohibited Countries
Encouraging Infringement Policy
Faces, Names and Signatures
Firearms, Ammunition, Replicas, and Militaria
Freon and Other Refrigerants
Government IDs and Licenses
Hazardous, Restricted, and Perishable Items
Human Parts and Remains
Importation of Goods into the United States
International Trading - Buyers
International Trading - Sellers
Mailing Lists and Personal Information
Mod Chips, Game Enhancers, and Boot Discs
Multi-level Marketing, Pyramid and Matrix Programs
Plants and Seeds
Prescription Drugs and Devices
Replica and Counterfeit Items
Satellite and Cable TV Descramblers
Stocks and Other Securities
Weapons & Knives
Wine (also see Alcohol)
Most of this is very obvious - of course you can't sell illegal things like drugs, pyramid schemes or stolen goods. Almost everything that is on the list is there because there is law against selling it. Some of the reasons, though, are a little strange.
The 'autographed items' entry, for example, doesn't mean that you can't sell anything that's been autographed - it just means that you can only sell it if it comes with a certificate of authenticity. The 'artifacts' entry prohibits you from selling Native American graves; 'celebrity material' means you can't sell unauthorised pictures of celebrities; 'embargoed goods' means that you can't sell anything that comes from Cuba… on and on it goes, and most of it you never need to know.
If your chosen kind of item seems to be on the list, though, and you're concerned that you might not be able to sell it, then check the full list at http://pages.ebay.com/help/policies/items-ov.html to see whether the item is banned entirely or there are just a few restrictions.
eBay says it will remove any items that it believes violate copyright law, but in reality they don't have that many people to monitor the site. You will generally only find that your auction gets removed if someone decides to report you - and even then, they might not get around to it.
Really, buying and selling on eBay can sometimes feel more complicated than it really is, thanks to all the rules surrounding it - not to mention the jargon. Our next email is a 'jargon buster', to help you learn the language of eBay.