Avoid for-profits. Many parts of the country have been infected with for-profit thrift chains like Savers. The quality of merchandise is lower (people are giving simply because their stuff has no value to them, not to benefit a charity), the prices are higher and you won’t get the satisfaction of helping a non-profit.
I'm not sure about Savers, but all the for-profit thrifts that I know of buy all their merchandise from charities.
For example, the Disabled American Veterans closed their store here years ago.
They still collect stuff through drives, donation boxes etc etc, but they sell what they're given to a for-profit thrift.
I know of a number of other charities and churches that do the same, saves them having to deal with all the hassles of running a retail store.
It's obviously varies greatly all over thec ountry, but I actually find far
better stuff at the for-profits, they put out stuff more regularly, and the higher prices discourage the big flippers.
BTW, there's nothing worse than seeing some ebay item you're watching with 2 days left and 11 total views ending up on the "put this on ebay roundup" .