If someone is interested in buying your book and they send you a message, you will receive an email at the email address you use for logging in on Facebook. Make sure you check that email first, or you won't get responses.

If you aren't receiving any messages after posting a book, there are three possible reasons for this:

1. No one has contacted you. This is the most likely reason! There are two remedies for this: either lower your asking price, or wait until the book will be used again! There is a very short window at the beginning of the semester during which students are looking to purchase books - so make sure the price of your book is fair.
2. You are checking the wrong email account. Make sure you check the email account that you use for your Facebook login. There's a pretty good chance that's where the message is.
3. The messages are going into your spam folder. Occasionally student exchange messages will enter your spam folder. If this happens, make sure you mark the message as "not spam" so you can train your mail provider to stop doing that.