Okay, so I've seen people on here say they use the waterproof priority mailing bags/envelopes from the USPS, turn them inside out and use those to ship books. I know it's illegal to do this with new ones, but what about recycling the already used ones? Is that allowed? If you just turn it inside out after receiving something priority mail can you mail media mail in it?

You cannot use anything that says Priority Mail on it, period. 

USPS can / will charge you (or your requestor) the Priority Mail costs for packages that have or contain Priority Mail boxes, tape, labels, etc.  You can't even use the mailers or boxes inside out (especially since Media Mail is subject to inspection).  

I know how frustrating this is.  I get Priority Mail every week and I can't recycle any of it sending books.  I feel especially bad when I can tell the box or envelope was used just once.  I have a lot of closets full of the PM shoeboxes and small square boxes holding all kinds of smallish stuff -- shoes, purses, mittens & scarves & hats, you name it! 

I also use them to put items to donate to charity.  Those have my address blocked out.

And, of course, I wrap presents using them.  My neices didn't know what to think when they received Xmas gifts addressed to me after they got the wrapping paper off.

Ange,

That's what I figured, but like you said I feel bad not being able to put them to more use.  I just wasn't sure because I've received books from eBay wrapped in these envelopes.

Thanks for the advice.

Other than priority mail, I reuse mailers all the time.  Manilla envelopes, bubble mailers, even inner plastic sleeves.  Just remove old postage, bar codes and addresses...if the new label doesn't cover them.  Less trash, and less for me to buy.