When a merchant posts an ad, he can set limits to the seller/buyer. The limits including binding your phone, completing advanced verification, finishing at least X transactions, etc.
If you meet the requirements of the merchant, you will be able to see the red "Buy" button and the green "Sell" button on the ads.
If you do not meet the requirements of the merchant, you will see the "Buy" or "Sell" buttons are grey.
Move your mouse on top of the button to view the detailed requirements of the merchant. You can place an order after you meet the requirements or you can choose other ads.