os buying help

i need help in buying an os off ebay for a computer i am going to sell. as i am selling the machine i obviously want to spend as little money as possible but still want the os to be legal (dont want to be selling dodgy goods!). basically what i want to know is what do i need to have when purchasing an operating system in order for it to be legal?

for instance a lot of sellers are selling a used 'coa' that is now free (taken from a now broken machine etc). if i were to get a copy of a windows cd would it be ok to use this and the official coa or must i have an offical cd also? any help on this appreciated and the operating system will most likely be win9x if that helps

You can't get a copy of the Windows installation CD. That is illegal, and called, "pirating". Although, you can get a distribution of Linux, which is gaining popularity. Linux is free for download. I recommend either SuSe, Redhat, Mandriva(formally Mandrake), or Fedora Core. They are the easiest to use, and you can download them for free. Listed below are the links to each of these sites:

Links:

SuSe - http://www.novell.com/products/linuxprofessional/downloads/
Redhat - http://ftp.redhat.com/pub/redhat/linux/
Mandriva - http://www.mandrivalinux.com/en/ftp.php3
Fedora Core - http://fedora.redhat.com/download/