If you're going to include an OS, you must include the CD/COA along with it.
If you don't have the CD/COA, don't include it and don't advertise that it will have an OS installed on it.
Do you have any
Heatware,
Beerology, and/or
eBay Feedback
Without any of these, you will probably be asked to ship first along with giving information along the lines as a phone number, address, etc . . .