Pretty much so, you can join up and put money in via Paypal, and then you make offers on cards, no less than 50% of asking price, you can also just buy them outright but not able to make offers, then pay via paypal, pretty simple...
I guess what makes the site different is that all cards are held by COMC at their warehouse.
So you can be buying off 20 different sellers but you will only be paying 1 lot of postage. The only cost to you is the postage and a $0.25 handling fee on each card you buy.
Ive used it a couple of times now and got a couple of hundred cards in each order. Its great, easy to use, sorting search results by different criteria (eg highest % off BV etc). Cant recommend highly enough.
I haven't sold on there, but just got my first order in as a buyer last week and was very pleased. Packed very well and the cards looked as they did in the scans.