As a former cast member of Magic Kingdom Guest Relations, things like this happened all the time before the reservation system.
I would never buy tickets from anyone other than the company. If purchased from a third party, the receipt you have is worthless as these tickets are sold at wholesale and it is almost impossible to find the specific ticket. It can be done in rare situations, but it can take a very long time. With the park receipt, it can be found within seconds.
Disney online shopping is also ok.
Never buy from third parties. You gamble and lose often. If someone sells a “4 day ticket” and it is actually a 10 day ticket that has already been used by someone for 6 days, it is illegal, the ticket will be confiscated and you will have to work with the police to get your money back. .
But even legitimate AAA banknotes, for example, are loose banknotes that are very difficult to replace if they get lost or have problems. You may need to return to AAA for replacements. It’s just not worth the risk as these things happen every day.
You might get lucky and save a little, or you might be unlucky and lose thousands. But your bet is not Disney’s responsibility to correct.
This particular person was lucky the company paid him back. It is very rare in my years of experience.