Branded OEM COA's are not legitimate if sold by themselves.  Dell, HP, etc. are not allowed to sell OEM licenses without a computer, nor is any one else.  Branded OEM COA's are to be distributed ONLY with new PC's assembled/sold by that particular OEM.  Any license purchased by itself must be a Retail license.  You may often see OEM media (just the CD/DVD) being sold by itself, which in and of itself is ok, as it does not include a license - it is just the media, however, Microsoft disapproves of this media being sold without its original accompanying hardware, which eBay attempts to enforce, so you may often see the media being sold with "some" insignificant piece of the original hardware (mouse, RAM, etc.); but if you see branded OEM media being sold with the COA, that isn't ok in any situation.
I volunteer on Microsoft answers forum concerning not genuine issues. Several websites are offering what appears to be Dell branded OEM recovery disks with a COA attached to the envelope. I have two questions:  where do they get these disks and the COA that is attached? Are they legal to use on any computer as claimed by the vendors?  It appears that the COA may or may not work. In one case the key printed on the COA was the OEM SLP key which implies a counterfeit COA.  I would assume that because legitimate retailers don't sell these products, these products are not for legitimate use?  Any light you can shed on this would help me give an appropriate response to posters with problems concerning these products.
Dell OEM recovery disks