Microsoft Registered Refurbishers can buy upgrade licenses for about $45.00 US. If you are buying an upgrade CoA, then that price makes sense but we have to submit the old CoA product key to pre-register that upgrade CoA with Microsoft before selling it as part of a PC purchase. This happens when we receive a Windows 7 PC and want to refurbish it with Windows 7 or upgrade it to Windows 8.1 in order to sell the PC. Other than that, I believe you are beholden to a retail license price from an authorized Microsoft distributor.
There are many rules for us in that program. I don't believe we are allowed to sell the CoA upgrade without a PC purchase since we are required to submit the old CoA product key into the portal, but I could be wrong.
I would be skeptical of the offer.