It really depends if I am not mistaken... The problem with steam keys [game keys in general such as fortnite], is that there is really only ONE way to get those without risking an account for the most part, and that's refunding something like G2A. However, it is a lot of work for someone to refund g2a constantly, swapping out pp accounts and what not doing various refunding methods over and over and having the risk of refund failing and them losing out on money.
With that being said, the problem that arises from the practice of selling game keys is people start carding, which is a totally different beast entirely. Carding GREATLY puts the account owner at risk, especially on the steam platform. Mpg staff has found that what MOST "sellers" ended up doing was carding keys for games and selling them off at discounted prices, only for account owners 1-3 weeks later typically getting screwed over as games were removed or accounts banned.
Keep in mind those selling 50-200 keys for account growth have NOT been noted for carding, those shops typically just amass a giant list of garbage indie keys that are easy to get and they sell them off here super cheap.
With that being said, if you can show a member of the MPG staff your method of acquiring the key, + amass a few vouches from very credible people in the community, I don't see why you wouldn't be allowed to sell steam keys in general. It's just touchy because of past experiences on the site.. [I personally have had 4 members of the community ATTEMPT and fail to scam me when I was trying to purchase some steam keys].