They will ban the seller's account instantly.
Absolutely not, in fact if you read and apply the eBay TOS and not give into their customer service's bullshit attempts at scaring you, there is one specific to sell CD Keys to the exact rule of the eBay ToS provided you didn't keygen and do things in the right order.
I made over a grand an got most of my domestic competitors banned around Thanksgiving/Xmas, you gotta just know when to call the bluff know that the law does not prohibit the transfer of a CD Key that has value and has yet to be used in a manner that's not deceptive, whether it be a gift or a sale, and if eBay asks you to put an actual photo of the CD Key as the top photo ask for supervisor or just hang up, but you'll be going the extra mile that almost nobody will, can, nor would be able to.
Between various accounts in various countries (some are trickier, US/UK is dead easy, in fact common law countries are easy generally), I made about $1500 in about a month and half selling maybe 4 or 5 or 6 different kinds of software. Unfortunately people want support and that ate up way too more time.
Oh yeah, total grayhat I'd say, if eBay asks to help them out..... if the person you talk to on the phone sounds confused just ask for their immediate supervisor and the number and address to the legal department, usually that serves as a good wakeup call if you do your research ahead of time. Your goal is to not only set prices but to get rid of all your competitors.