There's a 200 usd quota per month shared between products, services and currency exchange. If you buy something that exceeds your monthly quota it means that your quota for the next months will be affected.
It's a system designed so that no one who can save money can buy dollars. When poor people started selling their quotas the practice got quickly stopped with taxes that got its price above illegaly traded dollars.
So basically if you need to buy dollars here you are a criminal.