Selling "things" for money is not transferring money.
Selling stored value cards, like gift cards (which I think is the closest physical analogy), is transferring money and monitored by FinCEN.

Hamas is a designated terror organization by OFAC. US companies are prohibited by law from doing business with individuals or organizations on the SDN list. I believe Twitters user agreement defines the relationship as a business one due to advertising income.

TL; DR: I think Twitter is breaking the law by allowing Hamas to have an overt account.