You guys are missing something important in the argument about inspection of First Class and Media Mail. Only First Class Package Retail (e.g. pay at a postal counter, the retail rate) is not subject to inspection. First Class package commercial rate, which if you're using ebay, pirateship, stamps.com, etc... are commercial, and thus subject to inspection: https://faq.usps.com/articles/FAQ/What-are-the-Types-of-First-Class-Mail .
If you're actively avoiding MM due to possible inspection, you need to also exclude First Class sellers too; for cheapy books that doesn't make sense.
Priority Mail on the other-hand is not subject to inspection regardless if retail or commercial rate.