The site can't check who owns the domain being inquired on in the post GDPR world. In the incredibly rare instance that someone makes an offer on a domain that isn't Nat's he'll try to make sure your offer gets heard, or just refund the payment. It isn't his intention to be a service like DomainAgents, it's just an unfortunate byproduct of not being able to easily check if the offer is on his domain or not.
Some domains might not be for sale because they are being used for something, are on a payment plan, or something like that. It's pretty rare though.
The rest is CYA language for the rare occasions when someone files a PayPal dispute just because they couldn't agree on price. Haven't lost a dispute yet despite PayPal generally siding with buyers.
It isn't Nat's intention to take your money for nothing, just to stop the deluge of lowball and spam offers. You're a domainer so you should be able to confirm before submitting an offer that Nat owns the domain, and as long as he owns it you're 100% guaranteed to get a response. I imagine you can probably figure out if it is for sale or otherwise encumbered too. And you must know Nat's reputation. So I'm not sure what you're really worried about other than spending $19 and not agreeing on price. But that's the point of the whole system in the first place... to make people think twice before inquiring, not to earn $19.
I've been working for Nat going on 11 years now. If you want me to be the filter instead of the $19, feel free to PM me the domain, your offer, and your email address and I'll get it in front of Nat as long as it is a reasonable retail offer. If I decline to pass it along you can always pay the $19 and get the offer through to him anyway, so it can't hurt to give it a try this way first.
But unless you're representing an end user or are an end user for the domain yourself, there's almost no chance you're going to work out a deal. I don't recall ever seeing Nat sell wholesale.