Anyway, I guess what I'm looking for is the assurance that the essay I get won't be used again or even be borrowed from for some future paper.
If you're willing to trust them to honor their formal transfer of copyright to you, why wouldn't you just trust their explicit promise never to reuse your paper without a formal transfer of rights? It would be just as easy to lie about it either way and you'd be very unlikely ever to know they breached the agreement, especially if they waited months before reselling it.
Promises, contracts, and laws in general don't do much to protect you; they just give you statutory rights that you have to pursue on your own after a breach (or other wrong). Do you really plan to scour the Internet forever looking for a violation of your copyright so that you can then file a claim in court against a company and also make your use of their service public record in the process?
All the sites that use me all resell papers but they wait at least 3-4 months to do so except one, and that site charges more partly because of that (I assume) as well as because they specialize in PhD-level dissertations.
My advice is not to worry about copyrights and just go with a legitimate U.S. site that delivers a good product and won't rip you off. If you research carefully enough on this forum, you should be able to identify several. Good luck.