Ebay website is using auctions or direct buys. You can use a browser to access it or anything that's using a browser and an Internet connection. The payment processor is PayPal which means you will have to create an account on PayPal's website then create an account on Ebay. The PayPal accounts needs money or a credit card attached to. Once you have found the item you want, buy it or auction it and pay with the PayPal. Input the address and the seller will confirm you the payment and the ETA for the package.
