> There would need to be (1) an existing valid contract,
Your (1) is false. You can damage a business relationship that doesn’t involve a signed contract.
“Tortious interference with business relationships occurs where the tortfeasor intentionally acts to prevent someone from successfully establishing or maintaining business relationships with others.” https://en.wikipedia.org/wiki/Tortious_interference
They aren't doing it with the intent to damage his business. They're just doing something they would have done anyway.
You can't claim tortious interference just because someone throws a wrench in your business plans. Sanborn has about as much of a case as Microsoft has against Linus Torvalds for creating Linux and hurting their sales of Windows. (I'll give you this one for free: none.)
> They aren't doing it with the intent to damage his business.
That’s arguable. They sent him an email concerned about the harm of disclosure with the upcoming auction. They then apparently got offended by the offer of money to sign an NDA which calls their future motives into question as they now had a beef with the guy.
Saying the actions themselves were not improper is also a defense, and could be perfectly viable even if they had beef with the guy.
"To be improper, interference must be wrongful by some measure beyond the fact of the interference itself, such as a statute, regulation, recognized rule of common law, or an established standard of trade or profession."
They don't need a defense: nobody has yet stated a claim!
That's not a tort in American law. In this country contractual arrangement is required for tortious interference.