ThredUp is an online thrift store selling used clothing; you can buy clothes, and also sell your (very gently) used clothes to them. They also have some interesting grab-bags available sometimes, such as "5 random name-brand handbags" or "50 lbs. of random denim items, suitable for upcycling". They know very well when they've got serious designer pieces on their hands, so a very few of the "much cheaper than retail" prices will still get into triple digits for the really high-end stuff.

The nature of thrift shops encourages constantly checking for whatever new items have come in, and the site's marketing plays this up shamelessly. Be prepared to filter out daily coupons if you're on their mailing list. Knowing your size and measuring yourself accurately is also crucial. Thankfully, they are pretty good about often listing specific measurements of their clothing, and their return policy is quite forgiving as long as you're willing to be refunded in store credit.

ThredUP wants me to be someone who loves retail therapy a lot more than I actually do, but if you're determined to "buy nothing new" as much as possible, this is a pretty good way to do it while still having access to traditionally prestigious labels, if that's something you also care about.