If instead the exchange had been of real world money for N months of prepaid subscription, that was consumed after N months had passed, that'd be a little different but also presumably quite acceptable to accountants.
Suppose the exchange had been of real world money for N months of prepaid subscription credits that could be stored indefinitely and only consumed if the player chose to actively play during a month. That might turn into an accounting nightmare if those subscription credits didn't expire (maybe cannot recognise the revenue while they are unused, becomes liability on the balance sheet).
I wonder how the accounting rules work for stuff like Eve online where there is an game consumable item (PLEX) that when consumed extends your subscription, can be traded inside the game's economy, and can be purchased with real world money