I bought an adult pair of cal kings and both turned out to be females and both became egg-bound. When I emailed the "supplier" he didn't even respond. That pissed me off.
I also got a ball python from a respected breeder that was sexed wrong or mixed up and the breeder was more than willing to rectify the situation almost a year later.
"Buyer beware" is right though. Accidents happen. Animals can mistaken for the other sex and it helps to find information like that out right away so both parties have time to act on the situation. If you drive away from the MacDonald's drive-through window with the wrong burger and come back two minutes later, I bet you a million bucks they will do something about it. But if you came to the window and said "Um, I bought a Big Mac meal last month and you guys gave me a Fillet-O-Fish instead." I think you'd find it tough to get the problem fixed