The BW breakdown: Bottom line, Tradesy.com works and it's the fastest turn-around on listings that I've had out of all the other services you see here.
The BW breakdown: Poshmark.com is where I've had a lot of success with well-known labels. The look, feel, and usability of this 'mobile marketplace' is SUPER social. They are great at promoting products for you, love that. You take the pics, you list, and you wait to see who bites. The community is ridiculously tight, incredibly friendly, and they love to negotiate. There's flexibility in what you can post, it's all on your terms, and it's up to you to ship things out, which I can safely tell you is incredibly simple if you swing by a post office now and then.
The BW breakdown: Threadflip offers a bit more flexibility with what and how you can sell. You can go with the self-service or full-service (White Glove) option, depending on what you have and your needs. Certain labels sell better than others so I wouldn't go into this thinking everything will sell from Banana Republic to Tory Burch. Pick and choose your best options (big names; they have them listed on the site), send the picks over to them, and they'll cover the rest. You make 60% of the selling price this way. Or, go DIY and make 80% of the selling price if you have the time.
4. The RealReal
The BW breakdown: This service is best for those with plenty of high-end designer merchandise to sell that's in excellent condition (read: luxury resale). Also, to get the full service option, you need to live in a qualifying city. Basically, this is more restrictive to a large percent of the population but they curate an amazing assortment and sell through items like no one's business so, if you've got the goods, and can't be bothered with taking pics, sending things out, managing sales, then this is the option for you.
P.S. DON'T FORGET TO CHECK OUT CONSIGNMENT SHOPS IN YOUR AREA AS WELL!
*These recommendations are based on my actual use of these services or client recommendations and nothing else.