Please forgive me if this has already been suggested. I did do a brief search before posting.
Auctions for credits would give people who cannot afford to buy credits a way to earn some.
I really believe that most people who can afford to buy credits are probably doing so. If I'm right about that, then TS won't actually lose money on this idea. In fact, once people who might be able to afford a few credits (but who aren't buying any at present) get a taste for the delights of credit upgrades, they might decide to go ahead and part with some money. And again, people who simply cannot afford to pay-to-play will not be entirely excluded from some of the fun they are currently missing. There could still be subscriber only thinggies. I'm not arguing with that form of enticement.
I'm open to argument, esp if it is kindly done.