Here are the various kinds of software that you're not allowed to sell on eBay, and how to spot them. You should always try installing used software before you sell it, as this might give you a few clues.
This one is reasonably obvious, but pirate software can be hard to spot. The biggest things to look out for are auctions that don't have pictures or only have pictures from catalogues, unrealistically low prices, and come with no manuals or documentation. Microsoft software comes with a Certificate of Authenticity, which you should check. Software that is on recordable media such as CD-RWs is usually pirated.
If you see software that offered for download only, it is often pirated but not always. Some small software developers allow download licenses for their software to be resold on sites like eBay. Type the name of the software into a search engine, and go to the manufacturer's site to check. Remember that software you download is very unlikely to come with resale rights, though.
Beta software is pre-release software, issued by companies for testing purposes. Companies generally do not give permission for their beta software to be sold or redistributed, as it won't be as good as the final product. Do not buy anything that says it is beta, and return anything you receive that says beta or not for resale or distribution anywhere on the CD or during the install.
Some companies sell special educational edition's or student license's for their software, which are designed to make it affordable to students and teachers. You can only sell this software if you are a licensed educational reseller for the company, and your buyers are educational users. Academic software will usually say somewhere on its CD or in its installation what it is.
OEM stands for Original Equipment Manufacturer' this is software that is only supposed to be distributed with a new computer, as it came pre-installed on the computer. The licences for this software usually stop you from selling it without also selling any hardware. Look out for the message for sale with a new computer only, or similar. Note, though, that it's usually fine to bundle any OEM software you come across with computers that you sell on eBay.
If you sell any kind of software that isn't allowed, then eBay might shut down your auction, or your sellers might realise what they've bought and leave you bad feedback. It's not really worth the risk.