I noticed there is an option to gift a 1 month premium membership, but not to buy one. I think it would be a good idea to offer that option, as that's what I was looking to buy to test the waters.
I believe it has to do with the cash transaction fees not making it worth it, while if someone has credits it's just an internal transfer @ no additional cost.
fac vitam incredibilem memento vivere Knowledge Weighs Nothing, Carry All You Can