It is against Ebay’s EULA but isn’t illegal in and of itself. You could sell it on some other websites for sure. The people selling them on ebay will probably be caught and shut down eventually.
BUT, Basically if you have a really old account, any items you list will go live immediately (unlike with newer accounts) and the listings usually won’t get taken down automatically unless it’s a VERO item. As stated above, they will get caught eventually - whether someone reports their listing, or for any other reason eBay manually reviews their account (i.e. them asking for higher selling limits or something). If your account is old enough you could get away with a few but I personally wouldn’t risk it unless you have a throwaway account that you don’t care gets banned or not.
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Also it’s worth noting that eBay can remove listings that have ended, so just because you successfully sold the items doesn’t mean you’re in the clear - another reason why I wouldn’t risk it on an account I value.
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As your listings will be removed eventually. Its a matter of how fast your turnover. You sell big enough a qty, you do not worry about dispute cases or banned listings, because you have already made/withdrawn your money from paypal anyway
Also, have a look at this article… Just for an idea! 
Extras!
Selling Retail Keys Allowed?
https://www.aspkin.com/forums/auction-listing/15940-selling-retail-keys.html
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