I've done retail arbitrage with designer items and have used eBay, Threadup and The RealReal (TRR), I haven't used Poshmark.
Which platform I would recommend depends on the item and designer.
eBay will yield the highest percentage of the selling price (~80% after fees, etc), you are responsible for listing the item, taking pictures, writing the description, mailing the item. If you haven't used eBay for listing there can be a learning curve. I find that with high end designers (Chanel, Gucci, Dior, LV, Louboutin) the traffic isn't as good as TRR and if you have low feedback on your ID you may struggle because people are cautious about fakes. If you have more mid-range designer items (Tory Burch, Burberry, Kate Spade) eBay is my preferred platform.
TRR is easier but you will only get about half of the selling price (the exact percentage depends on selling price). All you have to do is send it in, they handle the listing, customer interaction, etc. I find that high end items get better selling prices on TRR. I think people are more comfortable dealing with a company vs an individual, it can be worth giving up the percentage for the higher selling price. My problem with TRR they are all over the place with descriptions and pricing, you don't get to set the price. I've sent in identical new items a few weeks apart the first one was listed and sold in the $200 range, the 2nd one was listed at $80 because the processor didn't look at the right key words. They have been resistant to change the price even with the documentation of the prior sales and I asked them to send the item back. Mid-range designer items end up with such a low percentage payout (I think it's ~40%) that I typically don't use them for those brands.
IMO Thredup has not done well for designer items for me, and I use them more for liquidation of more mainstream brands I don't want to deal with myself. They are also all over the place with pricing and their descriptions and photos are mediocre at best. Also for lower priced items they take a large percentage of the selling price.
Poshmark also seems to be geared for more mainstream brands (Anthropologie, LuluLemon) and like eBay you will be responsible for listing, photographing, etc. I have not used their platform so no advice on ease.
