Thredup is good, but just know that it takes forever and a day for them to process your items. When I'm ready to sell our family's clothing and other things, I go to both Mercari and Poshmark. Mercari's fees are cheaper, but I like Poshmark's website the best. I also like that shipping is a set $7.67 with the USPS (up to 5 lbs).
Also know this--unfortunately, it has been passed into law that if you sell over $600 in a year, you will receive a 1099 next year that you have to report on your taxes. Not a problem for me, but I can see where if you sell designer stuff for hundreds of dollars, that could be a problem.