The thrift store has never been about getting the items on sale to needy people that's just literally not the philosophy behind the entire concept.
The thrift store model has been about getting money from the people who shop there who are people who have money but not having to pay for the goods they sell because they are charitably donated to be sold and then using the money they make to advance the mission of the charity.
It's a fundraising concept. The funds come from people who have funds. The goods come from people that have spare goods. And then THE MONEY THEY MAKE goes to actually do good in the world. The limiting factor on every thrift store model I've ever seen is having enough customers. There's always tons and tons of stuff to sell, many have to throw away half the donations they get because there's no room.
The people who are in thrift stores buying things for Ebay and increasing the money that goes to the charity are HELPING the thrift store, not killing it. This mindless outrage is literally the opposite of the actual dynamic at work here.
thrift stores don't exist to help people find cheap stuff - they exist to raise or earn money, and many org's then use that money to fund the things they want to fund.
It might not be a 'side hustle' for some of these people, it may be the only income they earn, and good for them for showing their entrepreneurial instincts - and doubt very many are getting rich at it (except perhaps a few lucky ones that find that needle in a haystack).