You can't really buy anything online with cash. Some places used to take money orders, but I don't think they do that anymore.
The only thing you can use to buy things online is a credit/debit card or you can use a PayPal account.
Is there anything you can buy online with just cash? erm..don't think so. unless you use credit card or something
You need a credit card to buy pretty much anything online.. you can try craigslist.