I didn't apply stethoscopes to chests, but I sure didn't notice any birds that looked "off". So I am going to assume that the seller didn't know their flock was about to show signs of The Sniffles.
I felt a little less secure about some of the rabbits, and my 2 that came home with me are still in quarantine, after so many people went around touching every rabbit in the place and opened my cages without asking.
I don't know if people were going from cage to cage handling poultry. If they do that, then whatever that sniffle bug was it might have gotten spread around a bit.
I sold the pair of ducks almost immediately, so I didn't have any poultry and didn't see how the public was behaving around them.
If the sniffles showed up 2-3 days after the swap, the birds could have contacted it in the new home. Sometimes a flock has resistance but a new bird will get symptoms from whatever is in the soil.
However, a big yes to a notice that buyers are responsible for getting contact information and the sponsors of the swap have no control over what is sold.