First you will want to find out what generation iPod Touch it is. I'm assuming it's a 3rd generation (32 or 64 GB capacity) if the seller recently purchased the Mac. If it is a 3rd generation, then you will have no problems upgrading to iOS4 and taking advantage of all of its features. If its a 2nd generation, (but not an 8 GB) then you can still upgrade to the new O/S, but some features, such as the custom wall paper and multitasking, will not be available.
I was wondering if it had os4 on it?
Most likely not, but I guess it depends on whether or not it was recently purchased and if/or Apple has decided or started to ship its iPods with the new O/S.
Hope this helps. 🙂
 B-rock 