I have a few friends who resell, and the mindset is so different from those of us who sell here and there to clear out our closets. It's much harder work than it looks.
Resellers typically have dedicated storage and don't mind keeping an item listed for a year or more to wait for a collector who is willing to pay a premium. I personally typically get bored after a few weeks and donate the stuff. They also put time into researching comparable sales, taking great photos, and writing descriptions.
I know the math seems like the seller would be making a huge profit, but it's not so much when you break it down.
Buying the bag for $75 probably meant about $90 out of pocket.
If she sells the bag for $300, she'll pay $90 in fees.
She also needs to factor in some operating costs of her business, packing supplies, etc, so let's say that's another $10.
300-90-90-10= $110
The $110 will be taxable, and she will have tied up space and capital while she waited for a buyer.
So, the answer to your question about why these resellers didn't buy your other items is probably that their numbers make no sense unless they're confident that your price is about 1/4 of what they believe they can resell your item for.