WildCowboy said:
It may not be pirated software. He may very well have the discs and is just telling you that he doesn't have them because he wants to keep them and use them for himself on another computer.
That would make the software that the seller left on the hard drive pirated the moment the machine changed ownership; its the seller who is the pirate in either case.
The sale of software is generally only legal if the serial number and original media are transferred, and a transfer of license is made, giving you the ownership of the license, and revoking the seller's rights and interest in the software (including, he deletes all copies and doesn't keep those CDs he 'lost'.) Some companies require that you register a signed transfer of license form with them. A couple of companies charge a fee for transferring license.
That said, some software (notably any Microsoft or Adobe Educational or Academic version, and any software acquired under a site or volume license) that cannot be sold, transferred or given to another person under any circumstances.
To the OP: Consider this:
If the seller is prepared to cheat by copying software in order to enhance the saleability of their hardware, what's stopping them from lying to you about the machine to get you to buy it? You already established that they are a cheat...
Of course you can make a backup of the hard drive. However, you won't have the software installers, and many software packages require either reinstallation, or reauthorization with the serial number and/or original CD if they are copied from one drive to another.