1 star for selling, 4 stars for buying, for an average of 3 stars.
I've sold and bought clothes on ThredUP for about 4 years now. As a buyer, it's a lot of fun. Yes, they now add a restock fee, but it's relatively low compared to the money I save on purchasing clothes. That said:
SELLER BEWARE. Since I started selling clothes, they have changed their policies at least 4 times. To begin with, they paid for the clothes they took up front. Not a lot, but they paid. Then they paid up front for some, and sold others on consignment. Now, everything is on consignment and the return is literally pennies.
I NEVER send anything I care about to them without paying for return assurance (I have sent a perfect silk jersey DVF dress twice now and they won't take it - no clue why, you never do find out). I'm going to sell it on Ebay. At least with return assurance I got my very expensive dress back - I learned my lesson at the beginning when they didn't list an expensive bag and I didn't pay for return insurance (it was donated). Also, it's truly hit or miss as to how they decide to list things. Not worth the time or effort, IMHO.
Finally, as a seller, your branded dress that has 2 pieces (or a 2 piece suit) is sold as 2 pieces, not marked with the brand (many skirts or underdresses aren't marked when they are sold at retail with the jacket or 2nd piece), so no one has a clue what they're getting. My pretty 2 piece dress (black underdress with lace-up bodice) from Hearts & Roses? It's now listed as "assorted brands" and in 2 pieces. The underdress is so expensive it will never sell (it's plain because it's meant to be worn with the other piece (which I sent attached, btw!) and, of course, everything is consignment now so that's money that won't end up in my pocket. I've attached a photo of the 2 piece dress, in case ThredUP cares.
On this point, I've seen beautiful 2 piece tops (sheer, with a camisole) broken into 2 pieces. Good luck trying to match the top with the cami! That's a stupid way of handling clothing if you're trying to sell things, so the lack of care doesn't help ThredUp either. But I suppose from their POV it's all about quantity. Not to mention I've purchased a supposedly "unbranded" purse to discover it's actually an expensive brand. All I can conclude is that the folks who check the clothes in are doing it so quickly, they don't have time to really look the item over. That's unfortunate, because sellers like me are going to take our nice clothes elsewhere....
Bottom line: sellers stay away!