FBM Book pricing help

Hello all, quick question. I’ve tried to search this in previous posts couldn’t really find much about it. I’ve seen someone asking about how people make profit off selling books for absolutely nothing, but someone said it’s to get their account up. With that being said say books are ranging from 1-15 dollars I have been pricing them around 5-9 dollars. Is there a good pricing method that I’m missing out on? I just started selling books FBM this weekend. I’m not new to reselling at all, just new to fbm books. One last question, is it possible to get ungated in certain book publishers without having an invoice? Bc these are all thrifted. I have a series of books and they have a few different publishers and I’ve listed the whole series except for 1 of them which requires ungating.

TLDR: Best way to price fbm books