Somebody sells produce to his friend, and they do not discuss what the purpose of the produce is. It could be for eating. Let's say he sold him seeds that are edible but can also be planted. So maybe the purpose behind those seeds was for them to be eaten by the customer, or maybe they were for sprouting. Maybe he wanted to have them planted and have them grow as seeds. The buyer took the produce—the seeds—and planted them, but they did not sprout. They didn't give off anything. They were defective.
Even if the produce being sold was flax seeds, flax seeds are normally planted. They're not usually eaten, even though on a minimal level they're eaten. We say the seller is not responsible to pay back for the produce; he can always claim that he sold them to be food, not for planting. Even in the extreme case of flax seeds, which are used for planting most of the time, they still didn't work when they were planted. The seller could claim that he sold them for the purposes of eating, even though that's not usually done. We have a rule when it comes to money: we don't follow the majority to remove money from someone's possession. And so, even though the majority of flax is purchased for planting, the seller could claim that in this unique circumstance he had in mind that it was for eating. That's why the terms between the buyer and the seller should be very clear when the sale is happening.
That's talking about a type of produce that is interchangeable. It could be used for planting, and it could be used for eating. But if there's a type of produce that is used just for planting, and it was defective and didn't grow when the buyer planted it, the implications of the first part of the Mishnah would be that the buyer is obligated to receive a refund. And we'll say that straight up: Rabban Shimon ben Gamliel says that if the buyer bought garden seeds, which are never edible, like, let's say, cabbage seeds or leek seeds—never edible—they're meant purely for planting. And he planted them, and they never sprouted. They didn't grow anything from them. We say then that the seller is obligated to pay back the buyer because it's obvious in this case that they were bought for planting. And therefore, the whole sale is a Mekach Ta'ut (mistaken sale); these seeds were defective. This is only talking about a case where it's clear that the sprouting didn't happen because these seeds were defective, not because of the buyer's negligence in planting them properly, watering them, and caring for them. There's a Machloket (disagreement) in that Mishnah. It's just that Rabban Shimon ben Gamliel is expounding on the view of Tanna Kamma (first tanna). The first part of the Mishnah was talking about a case where the seeds were interchangeable. Rabban Shimon ben Gamliel was talking about where the seeds had one purpose, and therefore they didn't fulfill their purpose, and therefore the buyer is obligated to receive a refund.