eBay will ask you for permission, and you need to have a Microsoft reseller partner certificate. You can deliver the keys even through eBay messages. The funny thing is those approved MS resellers in ebay merge pirate keys with legal keys when delivering to customers. Therefore, eBay suspends accounts. So sooner or later, whether legit or not, your account will be suspended
The complete step-by-step guide to get back to selling today!
I have sold the odd Microsoft Office software with license over the years. No issues. You are allowed to resell your RETAIL software as the license can be transferred.
OEM versions, not so much. These are versions that are to be distributed with hardware.
If it’s a one off, should be okay. Just mention is no longer needed. Receipt included, etc.
It’s actually multiple one off experiences. Maybe a few a year. Now, if OP is looking to sell them regularly, I’d be weary otherwise, get Microsoft authorised status to be safe but I suspect it’s not easy if computing products and software isn’t their area.
Microsoft prefer to sell their Cloud subscriptions these days
Re: Microsoft Office DVD With Key Card 
