After you press the ‘Buy’ button you’ll see more information about the advertisement, including the terms of the trade. Read through them before submitting the trade request, if you don’t agree with them you can go back to the previous page and choose another advertisement.
To start the trade, type in the blue box how much you want to buy, enter a message for the seller and click the Send trade request button to the start the trade. Be sure you’re ready to pay when clicking the button, if you don’t pay before the payment window is over, the trade will be automatically cancelled.