The downside is that their most basic processing service charges $0.50 per card. So a box of, say, 200 cards is going to cost you $100 off the bat to list it. Plus you'll want to have a grasp on what you're sending in, a) to check for mistakes when they list it and b) because they require you to tell them how many cards are in your shipment, and accuracy is key.
Once they process a card and it's added to your account, and they take their $0.50 processing fee, it's available for you to list. You set your price, the card is live, and then you wait. Once it's purchased, you get the majority of the sales price - I think their fee is 5%. Sell the card for a dollar? You keep $0.95 in credit.
The credit sits there in your account to either pay for fees (there's a penny per month holding charge for any card you have listed over $0.75, plus any new cards you send in are subject to the $0.50 processing fee), use as store credit to purchase other cards, or you can cash it out and get it deposted/a check mailed to you - I think; truth be told, I've always left my credit in there to use for fees or to purchase other cards for myself.
I like using them because I quite frankly don't have the time to package and ship for trades or sales. It's unfortunate but it's a reality, and I'm trying really hard to downsize my collection. In this case, I just inventory, stick a couple of 100 count boxes inside a flat-rate box (or usually smaller USPS box), and send them off. I can list at my convenience, the majority of my cards are under $0.75 so monthly fees are minimal, and I don't have to worry about packaging or shipping. But there's definitely costs - 15K cards is going to cost you $7500 to list them alone.
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.