If they are significantly cheaper, they may be pirated. They could also be OEM versions that are supposed to be sold with a PC (but you should be able to register it).
There are a lot of free equivalents out there (e.g. Open Office, Lotus Symphony, Google Docs, Zoho Office etc.) but if you really need MS Office, then check with the seller that it is a legal version that can be registered and check that they have a strong rating (i.e. you can hopefully believe them). I saw a sealed copy of MS Office 2007 on EBAY for $199US and that is pretty good compared to the price at the Microsoft store for Office 2010 (about $650).