I suppose if you ensure that you'll be using this method for the purchase and that they're clear of the consequences, etc. that it would be accepted as an method for selling membership, to be honest, I'm not not too sure.
Summarised:
The Sellers Section only allows the sales of legal goods.
Seemingly as this is an legal good, I suppose this is alright.
I just couldn't find anywhere stating that using an somewhat illegal method wasn't allowed.
Anyway, probably something to consider asking staff.