Lloyd,
The different levels of "proof being in the pudding" would start with actually walking into an attorney's office, spending more than the initial consultation fee, and maybe even putting down a retainer, and having that attorney get up to speed on the "facts" of the case. As you know, the attorney will raise many of the same issues and "yes buts" in analyzing the different thresholds in the case. Not saying the case can't be brought, and doesn't have a prayer of succeeding. At present, both with Allen and any other similar scenario, I do see it fraught with difficulties. I think that many of your "cut and paste" points were touched on yesterday prior to your post. But on a couple of points:
BTW, I've already mapped out the legal argument in the last post, he is on notice of a problem in his collection, it is arguably negligent to sell under these circumstances.
And arguably not, as he may be able to show that he has quarantined one group from another. I am aware that IBD has managed to spread despite the best of quarantine procedures in hobbyist collections. Point being that the spread of the disease does not necessarily indicate that the vendor did not take "reasonable" precautions, especially a wholesaler of cheap animals. This may be an area that health statutes eventually get a hold of, so that they can be summoned to test at a facility, at random or because of a complaint, and if they find IBD, they then have some legal authority to mandate a protocol. But as of right now, there are no state regs, and certainly not industry accepted ones, for low-end wholesalers. They all have snakes on top of snakes. Whatever steps a vendor may take, the door is still open that they have not eliminated the problem. Somewhere between doing nothing, and doing quite a lot, if the disease still moves out in a sale, a judge or jury will have to decide where the threshold of negligence was, and if it was crossed.
Since individual animal quarantine is still the recommended method, and this being primarily the obligation of the buyer, should it spread from a single animal into many owned by the buyer, the defense will argue that the buyer was negligent for the spread.
I agree that if it can be shown that a seller sold an animal that was known infected, or housed directly with a known infected, its a slam dunk. An easy case would also be the "dumping" of animals rather than attempting any quarantine and control once a vendor is "on-notice". Certainly, anything that can be shown to smack of a very "unethical" approach would support negligence. But there's a huge grey area out there for now, and even in the pretty air-tight of civil cases, 70-30 is often the best estimate you will get of prevailing. Add any smidgen of ambiguity, and down it comes. My opinion is that at this stage of the game, you'd be hard-pressed to get even a 50-50 assurance of prevailing.
Then there's the costs. You may prevail to some percent of your claim, and be unable to get any of your costs covered ..... aka spending $20-30K to win lesser judgement.
It is also possible that a well-heeled plaintiff could bleed a vendor-defendant into some degree of submission. That would only create a settlement, not a precedent.
I would agree that, in-time, things will happen. Just not tomorrow.
The point of asking to see a successful case was to show that there hasn't been one yet to serve as a roadmap, and to imply how easy it would go was a big leap. The TSE situation demonstrated to many here that there is a significant threshold between fairness and justice, as until someone has actually been there, they often underestimate it. 
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