Is it normal for there to be a nonrefundable deposit (say, $500) in addition to the assignment fee?

(as well as an escrow account for deposits...).

If they are a first time renter I require a cosigner or a double deposit. 

If this was a standard rental process then yes, I'd recommend going through the process of applications/screening steps, a background check, and then require the deposit; since it sounds like the house is being rented to transient workers though, the very nature of the work situation calls for a bit more flexibility on the renting end.

That just drives me up the wall....BUT ANYWAY - to answer your question: You can walk away from he transaction but depending on how your contract reads you may be forfeiting your deposit.

I just formed an LLC, opened a business checking account, and signed up for Stripe to take credit card payments that deposit into my checking account.

As you will need to put money down as a deposit and do some Due Diligence costs.

I would take out ALL metal plumbing and update waste pipe to PVC and supply to pex.HERE, we have a lot of rust, calcium deposits, and lime, besides grit ruining faucets etc. 

I have a CPA whose clients have the tenants send him the check, he books it and deposits it, and no need to get licensed as he explained, as I'm just a messenger.

So, the things that stand behind it such as a deposit, proof of funds, etc make the seller know that the promise isn't idle and has a strong likelihood of closing.