If you're going to sell and buy over a phone use an encrypted app. Get all your boys on it and be smart.
T elegram - Such a sick app. 'Secure chat' encrypts all chats and erases all logs on both parties phone. The Gifs are awesome as a bonus. There is a self destruct timer which can be set different per convo. Ie After 24 hours self destruct both users chat.
The company itself is open source so we can trust it and it has no back doors. If your device is taken and you did not use secret chat your logs may be recoverable so bare that in mind.