Swede here. My guess is that it's mostly cash but I wouldn't be surprised if some of the transactions goes through the very popular big banks-backed mobilenumber-addressed instantaneous app-based no-fee bank account-to-bank account transaction system Swish.
I don't know about Sweden, but in Copenhagen the only times I've seen a queue at a cash machine is the one nearest Christiania, i.e. where cannabis (and more) is sold.
I'd be very surprised if a drug dealer accepted Swish/Mobilepay.
