I don’t think you’re making the point you think you are. Just because it’s how something has always been done doesn’t make it right.
And even within a strictly capitalist framework, there’s a world if difference between making a profit (even an extremely handsome one) because you are providing a service by uniting a seller and a collector; and making a lotto sized windfall based on insider knowledge and keeping the seller intentionally in the dark. There are laws against investment trading based on inside knowledge, and against profiteering over captive or vulnerable consumers. Why should a garage sale be any different.
Arguments based on " well, the seller should’ve done their research" or “it’s not my responsibility to educate anyone else” is just justification for theft. It’s no different than if I just go into your house and take the undervalued article in question. How do you think me saying “you should’ve done more research on how to protect your possessions” or “it’s not my job to point out the flaws in your home security to prevent me from entering and taking whatever I want” would go over in court?