If it's used what OS does it have? I'd leave it as-is if there is an OS. If there is no OS installed, are you intending to pay for a genuine version of Windows to be installed? Seems rather a strange situation.
If you are selling locally, the buyer will want to see it working. Who would buy a PC that does not do anything from a stranger?
I was planning on buying a Windows7 OEM System Builder OS for it and have them install it there selfs, but some people like to choose there own OS. Thats why I am not sure what to do.If there is no OS installed, are you intending to pay for a genuine version of Windows to be installed?