I sold some collectibles on Steam to a guy for euros, pretty sure that the only reason it wasn't a taxable was because it was a one-off and under the reporting minimum.
I would figure that the conversion to cash would be the taxable event. After all their value is largely indeterminate (and at that point 0) while the collectible is being given out.
IIRC this created problems for Blizzard when they did the real money auction house on Diablo III, and was part of why that was ended.
I agree for assets like those, the point is that for many other assets (art, Bitcoin, cash, stock), receipt of the asset for less than market value is also taxable.
But those usually have more developed secondary markets where prices can be widely known (and also nontrivial value). I can at least see the argument.
