If you are talking about buying and selling articles, then there is a section in the forum called "Marketplace". There you can sell your goods (physical or digital) and get paid in bitcoins. Bitcoin is a money and hence it can obviously be used for buying and selling stuff. Also in your local area there may be merchants accepting bitcoin as a payment - though this is rare. Another thing is the Darknet where bitcoin and crypto trade is rampant and I do not suggest you to join that,