Why traders buy humble monthly gift Links for PayPal and csgo keys in steamtrades?

Example: Total War Warhammer was 12$ on HB, and all were traders were buying it for 7 csgo keys, cuz they can easily sell TWW now for 25$+ on grey market(they need HB giftlinks to be sure that keys will work)
EDIT: They buy for csgo keys or paypal , cuz sometimes they can't buy multiple copies of same bundles

They resell it by the bulk on Kinguin and G2A. Depending on game, demand, and luck, their profit rate on the bought key can be up to 200%. This is why you can see some of them pay now almost 20 bucks for TW:WH, even if that game cost 12 bucks for the original HB buyers. That early access survival game RUST had a similar markup (IMO proving that their player base is as dumb as a rock, but that is another tale).

Before you ask then why people who sell them don't do the same: the fees are actually enormous unless you do the trading operation in medium-scale, so you'd be forced to buy up leftover links yourself. (Also, not everyone would disclose their payment data to a company that is yet to show any record of following laws.)