I think your forgetting that the $100 being spent is going into the T-Mobile account that would've been spent anyway. Anything more (like the $50-100 CB) is pure profit.
My question was why it's worth it for someone to "sell" the credit if they don't make any profit.
The asnwer was that there are two offers. One of them is profitable (and therefore people are selling it) and the other isn't profitable.