"Normally, the OEM version of the software can not be sold by itself..."
Under the (recently amended) OEM license for XP you COULD sell XP OEM WITHOUT any hardware - however the OEM copy had to be installed on an ALL NEW PC. You can't just add a new hard drive and install an OEM copy.
Can't find out if the same applies to Vista OEM but suspect it might.
"The OEM version lacks the retail packaging as well as the product manual..."
It also lacks certain support from MS which some customers might want.