you make an account on an exchange, buy btc, then send it to the address the seller gives you for the order. buying btc can take awhile and the exchange will only let you buy a few hundred dollars worth in a day because of money laundering rules. they want you to send btc because payment processors won't let them stay merchants due to the whole selling drugs thing. iirc coinbase will let you use a credit/debit cards after an id check (obligatory reminder not to use crypto for anything genuinely illegal because it's not anonymous - estrogen is luckily gray market).