Interesting. This exact thing happened to me several years ago when I was working with Pueblan milk snakes.
I had sold a girl a "pair" of babies, and a couple of years later when she went to breed them, discovered that the female was actually a male. Oops! Now of course she was not happy about that, but what do you say to her? I certainly was not going to surrender one of my breeding sized females to her in exchange, and certainly was not going to refund her the price of the animal PLUS the feed and maintenance over the last couple of years. So I just started off the conversation asking what she wanted to do about it, with those caveats understood. What we agreed to was that she would meet me at the following show (I think this was at Expo when in Orlando) and pick out whatever she wanted off of the table as a replacement. I don't think it really satisfied her, but she did realize that there is a limit to what someone can and will do in a situation like that.
Missexing happens to everyone, regardless of experience level. However for a new purchaser, this fact is not really driven home until they reach the point where they are actually sexing their own animals. Most do not realize that this is not as simple as looking at a blue or red dot on the snakes' head. Any interruption or lapse in concentration can do it. When I sex my snakes, it is normally because I need to do so before the show coming up the following weekend, so necessarily I am in semi-panic mode trying to sex several hundred babies and doing this 3am in the morning. This is about a worst case scenario as you can get for concentration. I have caught myself popping a snake as a male, but writing the female sign on the ID tag. I don't catch all of them, certainly, when I am doing it. I have also found deli cups sitting on the tables at the show preparing for opening of the show and noticed that the name label says it is a male yet the penciled in notation on the ID tag says it is a female. I just grabbed the wrong label by mistake. I have had people ask me to verify the sex of an animal at a show, so I pick up the "female", and even a quick glance at it visually tells me it is a male. Popping embarrassingly verifies that fact. Heck, I learned a long time ago to ALWAYS write the sex of the animal in pencil!
Anyway, this just happens. People buying snakes (or any animal where sex is not obvious to determine) need to realize this and verify the sexes themselves of the animals they receive. But this in itself will introduce yet another problem. When you have an inexperienced buyer sexing the animals they just got from you, telling you that YOU were wrong, how do you resolve this problem? What happens if you get it back and you find that you actually did sex it properly but the buyer made a mistake? I doubt anyone will think such a scenario is unlikely to happen.
What can be done about it? Nothing much except verify the sexes of the animals you sell as many times as you can. I sex the babies when I mark the deli cups, and I also try to sex them all prior to shipping them out to a customer. Even with this, I am certain mistakes still get out the door. Even with such a miniscule percentage of error of 1 out of 100, with 3,000 snakes going out you STILL have 30 errors that someone is going to be unhappy about. Get REAL anal about it, and check and recheck the sexes, maybe cutting that by half, you STILL have 15 pissed off people somewhere, sometime per year.
When I pop the babies, I try popping them three times at a sitting if they appear to be females. Meaning, three tries without a pop means it is a female. Now the scary part of this is that in some instances, I have had babies not pop on tries one and two, but pop on try number three. How many of those that did not pop on the three tries might have popped on the fourth try? But when do you stop trying? When you have crushed the tail to a pulp? This is fraught with potential error anyway, because in most cases the act of sexing a snake is trying to prove a negative. Everything we do is basically saying "well it doesn't appear to be a male, so it must be a female." Truthfully, the ONLY 100 percent guaranteed accurate way to determine if a snake is a female is if it lays eggs it is a female. Probably.
