Hunter65536: One doesn't need to store much right? I mean 25$ is more than sufficient for such purposes and that's hardly a loss compared to the money you would lose by losing your account.
Vythonaut: Even $5 is too much in my opinion. Don't look too close; it'll gonna be $5 from me, $5 from you, $5 from max and so on.. ;)
If you can only use the credit to buy games, it'll have to be games costing no more than that, can't transfer the credit of more accounts to one. Also, if you can only use them to buy games... I guess you could buy and then resell, especially if from one of the countries with discounts, but this should probably be caught quickly, and shouldn't be that hard to implement optional security features against it. Start with 2FA for those who want it, then e-mail confirmation required if logging on from a different place (but no blocks as other places have, just confirmation, possibly optional 2FA confirmation requirement in this case even if that's not enabled for everything), confirmation required for purchasing gift codes, and visible activity history (login attempts, purchases, downloads). And users will then enable whatever they feel is needed.