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 a few prohibited or questionable items:
Alcohol
Artifacts
Autographed Items
Bootleg Recordings
Brand Name Misuse
Food
Freon and Other Refrigerants
Gift Cards
Hazardous, Restricted, and Perishable Items
Human Parts and Remains
Offensive Material
Pesticides
Prescription Drugs and Devices
Recalled Items
Recordable Media
Replica and Counterfeit Items
Stocks and Other Securities
Stolen Property
Tobacco
Travel
Unauthorized Copies
Warranties
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.
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.
Next is a 'jargon buster', to help you learn the language of
eBay.