I just received my first bad book. It has the corner of the cover missing on the bottom right corner and the first 75 pages  of the same corner look as if they were chewed. I went through all the prompts. What happens now? I will not be able to post this book again. Will I get my credit returned or am I out of luck/

In order to get your credit back, you need to PM the member you got it from explaining that you would like a credit returned because of the damage (and it never hurts to quote the Help Center where it states no chewed pages). The sending member is not obligated to return the credit so be polite and respectful when PMing. Good Luck!

The corner of the cover can be missing up to about 1 square inch and the book would still be postable.

If the page corners are just beat up, without being ripped or torn, then the book is probably still postable, as well. Curled and bent pages are postable. But chewed is not, so if they were really chewed, or ripped ot torn, then the book is not postable.

In your message to the sender, you should be clear about the response that you want from them. If you want your credit back, you should ask for it in your PM message to them. If they do not respond to your message within 5 days, then you should follow the directions in the help center and mark the sender as not responding to PMs, and PBS will prompt them to respond. They do have the option of returning or not returning your credit, though.

PBS cares that the othe rmember answers your message, but they will not get involved in mediating the actual transaction, as they have never seen the book, and anything that is said about it is he-said/she-said.