I’ve used ThredUp too and it’s a very different model to TTE (also not a horse business so not really comparable anyway). ThredUp takes basically anything, including stuff that’s old or not in great condition, and relies on fees and volume to make a profit. Most of the stuff they’re selling wasn’t worth a ton when it was new so not going to bring much in used either, but it’s still better than nothing.
TTE is pickier about what they’ll take so it’s mostly higher end stuff that offers larger profit margins even used. They lay out their payment plans very clearly on their website so you can see what percentage of the list price you can expect to get.
Consignment really isn’t about turning a profit, it’s about getting a slight return on investment while clearing out stuff you don’t need anymore without it all ending up in a landfill. People way overestimate how much their used stuff is worth and then get frustrated when no one wants to pay that. I worked consignment as a teen so I’ve been on the receiving end up plenty of those complaints.