I appreciate that you didn't ask for our opinion, but: just don't! lol
XP goes out of extended support in April, that means no security updates, zero day exploits and exponential incompatiblity. It will be the end of third-party software developers writing things to support it.
If you're rebelling because it's too slow then the system could need more RAM or a faster hard drive
(SSD is always going to make a significant difference
)
If it's for legacy compatiblity, Virtualise XP on there but keep the main OS supported
If it's because you don't buy into Microsoft's increasingly incoherent vision, Debian or Ubuntu will set you free
The danger of buying eBay XP is that you don't know what its activation history is. Media Centre Edition as mentioned above by Rich was OEM only and if it has been
(re
)activated recently
(last couple of years
) Microsoft will have the activation on file against an OEM license key. The online activator could simply turn around and refuse to allow it.
If you do get it from eBay, get a retial or retail upgrade version. At least that way the key can legitimately be re-used
(but again if the owner has activated it in the last year or so you could have problems
). That leaves shrink OEM or Retail - which Microsoft no longer print.