If this is a new problem, you’re probably suspended. Suspended users can still use the app, but not socially.
Most suspensions are temporary, but in some cases your suspension may be permanent, depending on the cause of suspension. Email us at email@example.com to ask about the duration of your suspension.
If you’ve never been able to sell or comment, then you may not have verified your account. If you try to comment and the app prompts you to verify, you can choose to send another verification email to your email address. If you tap the verify button in that email you can start to comment and sell.