Earlier quoted context omitted.
I was in Cuba this January, and I didn't get the feel that this was illegal. We bought the cards on a busy street in Havanna with no problems at all. Also, we paid 2 CUC and were told the official price was 1 CUC.
I did the same last October. It's definitely illegal, only ETECSA kiosks are allowed to sell the cards. But resellers are ubiquitous and it seems that the authorities mostly turn a blind eye.
EXCEPT when they want to destroy your life or make you suffer for any reason, then they'll look at you specifically.