I came across a neat emergent phenomenon in SWTOR: player bankers.
A non-subscriber in SWTOR has a credit cap of 350k credits. Once at the cap, any credits she gets above that amount go into an escrow account. She has to purchase escrow unlocks, either through the cartel market or on the auction house, to access any credits in the escrow account.
One method players have come up with to evade this restriction is player bankers. The banker is a subscriber, who can hold unlimited amounts of currency. When the non-subscriber approaches the cap, she deposits a significant sum with the banker, dropping her back below the cap. The banker keeps track of her account out of game, perhaps in a spreadsheet.
When the non-subscriber wants to purchase something expensive from the auction house, the banker acts as her agent. The banker purchases the item for her, and deducts the amount from her account. Usually the banker takes a fee at this point. Since the fee is less than the escrow unlock, the non-subscriber comes out ahead.
One thing about SWTOR F2P mechanics is that almost all unlocks are tradeable items, which can be bought and sold on the auction house. This is the feature which makes the banker a viable agent.
The transaction is also smoother. The non-subscriber doesn't need to juggle escrow unlocks, which come in specific sizes. She can make very large transactions just as easily as small transactions. I imagine that the banker can act as an agent for selling expensive items as well.
Of course, the downside is that the non-subscriber has to trust the banker. She has to trust him not to make off with the credits, but also to keep subscribing and keep showing up online. So most banker relationships occur through friend or guild channels.
I think it's an interesting phenomena throughout, and matches the historical emergence of banks to a degree. I also think that the more sandbox-style games can learn a lesson from this, and see that it might be possible to offload currency and monetary systems onto the players' shoulders, rather than insisting that it is handled by game systems.