Allowing trading is a big part of it. Most online games never allow trading the things bought with real money, they get tied to your account. I guess as a way to prevent CC fraud but it still contributes to the issue.
Trading wouldn't work due to online game deflation. They have to set you up in order to retain you. When you open a new account, or are a "returning player" you get a bunch of free/easy to get stuff that took someone else a decade to collect.
It's a double-edged sword. For the seller, the ideal would be getting people just as addicted but not allowing trading, since that increases the average spend required to get a specific desired pull substantially.