Part of this is getting into the etiquette of soft-sales and the unspoken but established rules go all the way back to the days of classifieds.
When it comes to buying that stuff (e.g. Facebook local lots, Offer Up, Craigslist, newspaper classifieds), the rule has always been, in my observance that it’s first come, first serve.
Now, if someone shows a lot, agrees to a price, and then they sell an item or two to someone else, it’s their duty to inform you that parts were sold before your pick up, and it’s customary to either counter with a cheaper price, or ask the buyer if they are still interested in working something out. It sucks, but there’s nothing wrong with that as it kind of falls in that first-come-first-serve jurisdiction.
But, if this was an eBay thing and someone showed a lot of 100 NES games with a Samson or Dino Peak in there, and you get the lot and they are NOT in there, then that is fraud. It’s false advertising and lying.
Even if the lot was a good deal otherwise and I needed 75% of the carts in the lot, I’d want a full refund and for it to be shipped back at the owners expense. It’s not about being bitter or getting back at them. It’s just that if you’re going to try to pull something like that, you don’t deserve my business and, hopefully, loosing money on the sale will teach them a lesson.