If Iran can't sell oil, it cannot buy
gasoline. Little known fact: Iran lacks substantial refining capabilities in its own country to make gasoline. They export oil, import gasoline.
Because they can no longer export oil, they won't have financial reserves (cash) to import gasoline.
That's not going to make the masses in Iran happy.
Iran can sell plenty of oil to China, who isn't picky where they buy it from. They can buy gasoline through straw men wherever they want. Their own refinery capacity has been expanding at 20% per year as well. Iran's oil problem is declining production, not boycotts.