Lessons to be learned here...
As a seller:
#1 - Never sell to a minor without an adult present.
For anyone really young I wouldn't sell to them without a parent. I don't know anyone at shows that ID's younger people before they sell a reptile to them unless it's venomous.
Well, I'm not saying necessarily that it needs to be done this way, but this is the way I look at it... Most pet stores won't sell animals to minors. There's a very good reason for that! Why should reptile shows really be any different than the reasons why most pet stores adhere to this code? In fact, people, especially kids, in my opinion, are more apt to make impulsive purchases at a reptile show than they would at a pet store, primarily because it's such a fast paced selling environment and people don't usually get a chance to really reflect on a particular purchase, especially if they fear it's going to be sold within minutes. So people make purchases on the fly and that's when mistakes can be made.
As a seller, I think it's especially responsible to have such an edict; it shows that you are looking to curb impulsive purchases made by children as well as making sure that the parents know what their child is in fact purchasing. How many parents out there have let their kids loose to purchase whatever they want and ended up having a pet that was ultimately unwanted? Of course, having said that, it also ensures that you care about who your animals are being sold to.
But that's just my take on things.
#2 - Always openly disclose (and mark on container) any known abnormal issues with the animal.
This is what the dispute is mainly about that this isn't really all that abnormal in large female hognose and is actually somewhat common.
Now, I wasn't specifically stating this about the overall gland issue. I was merely pointing it out because you actually did say "The one thing I did do wrong with this sale was forgetting to mention the females tail. Had I remembered to mention it before the sale I would have.". Having said that, there are always going to be those animals that have abnormalities that should be noted on the container they are in. It's too easy to forget bringing it up and of course it's too easy for some buyers to initially overlook. Ultimately, it's up to the seller to disclose all and any information that would be pertinent to helping the buyer make an informed decision. You dropped the ball on this as you already said. But...the buyer did too as they should have carefully examined the animal before even purchasing let alone leaving the show. Again, lessons learned!
Now, about this whole gland issue... I have about 140 Western Hognose in my collection, probably 30+ of those being adult females, some being 5-7 years old. Perhaps I'm lucky or maybe I've not kept enough hognose for longer than some people to have witnessed this, but as far as my collection goes, I don't have any females that have ever shown this kind of swelling. And contradicting what Justin claims, I keep mine on solely on rabbit pellets, probably one of the dryer substrates out there being used, and again, I've never encountered this issue. I'm not saying it can't happen to any of the animals in my collection, I've just not seen it yet after almost 7 years of keeping and breeding Western hognose.
So, by saying that it isn't all that abnormal and actually quite common, I'd have to seriously question that as well as pretty much everything Justin has claimed. Perhaps others can chime in on about how common this is in their collections; I'm very intrigued by this claim!