Genetics is not the same as a health issue, Damian...as was pointed out by Seamus, they don't change. I have never sold a het animal I didn't produce, so I don't have to worry about that situation.
I'm sorry you took offense to my comment about your business - excuse me, your hobby - but as I said, the tax portion of my post was a side issue. I only commented further on it in my second post because you chose to focus on it. I posted in the first place because you initially seemed to be implying that because you were not a business, just a hobbyist, that you held no (or less) responsibility. The declaration that you were a hobbyist spawned some discussion about what your level of responsibility might be. IMO, regardless of whether you are a functioning as a business or a hobbyist, the expectations should be the same.
I have not commented on the sale of the hets (or false hets, as the case may be - through no fault of yours), and what you should be expected to do about it. I don't feel that you are under any obligation at this particular point in time. Should your customer contact you and inform you that the hets did not prove out (and there have been enough offspring to reasonably accept that they are not going to - a normal factor which can likely be omitted given the origin of the animals in question), then as the person that sold him the animals, you would be responsible for making good on them. I would not hold you to any expectations that I wouldn't hold any other hobbyist or business to. Some people may have outstanding memories and/or records, and be able to trace the origins and destinations of each animal that passed through their hands several years later. I suspect that many others from both groups could go back and say they bought hets from persons A, B, C, and D in this year, and A, C, D, E, F in the following year; and they could likely identify which customers purchased hets in those years...but I don't know that they could identify who purchased animals from specific sources. Again, not dealing with hets, personally, maybe that thought is way off base. Regardless, unless it is expected that everybody that sold a TSE het is going to seek out the purchaser and offer compensation, I wouldn't expect it of an individual - in this case, you. Sure, if a person had a finite number of TSE hets pass through his hands, contacting the purchaser is the right thing to do...but, IMO, it would be appropriate to inquire if he still had them, if they bred, and if they produced what was expected, prior to making any offering. Based on the answers to those questions, it might then be appropriate to discuss the origin of the animals, and the possibility of remuneration. It is commendable that you are willing to search out the individual that purchased those animals, instead of waiting for the complaint.