That really depends on the type of product or service you're buying and whether the seller is willing to accept cash. A seller may never deal with cash and, in that case, you wouldn't be able to pay with cash because he/she simply does not accept it. On some websites there may only be a shopping cart system which accepts certain forms of payment, such as credit cards, so payment is limited because of the tools or scripts used. Some sites may accept cash. I believe Ebay allows that option. However, it's up to individual sellers if they want to accept it and generally some sort of shipping insurance or tracking number is preferred, in case cash is lost. It's generally safer to pay with a check or money order, both of which can be better insured or replaced if lost (or stolen!). Cash may also not be an option if you're dealing with international sellers.