PayPal is the typical method used by most folks on the classifieds. It streamlines things and take a lot of the risk out of buying online if you pay as "Goods/Service". Usually the buyer pays the extra 3% as they will be the one protected.
I am the official and only Purchasing Director. I act as a go between, to make sure all sales go smoothly, and no one gets ripped off. So when anyone (like yourself) wants to purchase something from the classifieds here, they need to send the funds to me. I confirm shipment and then release the funds to the seller, upon purchaser confirmation of receipt of the item(s). In short, I accept PayPal, bitcoin and credit cards.
Okay, I was only kidding. The guys above covered it.
Usually the seller puts info in their ad about method of payment. I guess I'm still old school and don't have a Paypal account, so if I'm selling, it's either a personal check or USPS money order. Same thing if I'm buying. I've never had a seller refuse either of those payment methods. In fact, I even once sent cash for a small item where I didn't want to mess with a money order. I prefer to get a phone number from a seller as well so I can call and chat if I deem it necessary.