Exactly. If you're selling stolen or pirate product at a cut rate, people will tolerate goofiness for the better deal, but if you're scalping and people are paying above-market, they're not going to take the risk on sketchy purchases that may get their Steam account banned or something. And the scalper isn't going to take the risk buying a ton of product they can't unload, so even Valve stating they will enforce this would have a measurable effect.
replyImagine buying the SM for a teenager in your family that doesn't have a Steam Account yet.
replyFirst: Imagine a situation that won’t happen. A teenager without a steam account?
replySecond: They could easily allow any account to log in as long as the account that bought it is an actively logged in profile.
If you think that 'teenager' is too old (which I am skeptical about, considering the current popularity of Roblox), replace by 'kid'.
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More complications, more points of failure.