There is multiple options to get payed, depending on the country there might be different methods so I'm not quite certain how would that work in your case.
In our country (Poland) typically we have PayPal, bank transfers or post office money transfer. For first two you need a bank account, the last one doesn't require it. In that case you fill proper form while sending the package to the buyer and he pays for it when collecting then few days later you get a notification so you can get that cash at the post office.
Btw those trading websites usually require some kind of registration and confirmation of your identity. Once you are done with that you can buy or sell as long as you want. Some of those trading sites also charge fees for the seller but typically when the trade is done successfully. Those fees also may vary but are like 1-2% value of the sold goods.
Please specify your country then maybe someone from there will confirm the payment methods and provide more informations about some specific trading sites in your location.