Id love to set up a branch of COMC here in Australia that is in direct contact with the US one to make it cheaper and easier or Aussies to sell to US etc..
anyways,
As a buyer,
You go shopping and find whatever they have for sale that you want or need, you can but them outright for the listed price and add them to your cart or you can make offer on cards to get hem for a better price ( you need to deposit funds first to make offers)
you can keep buying for days, weeks or even months, and keep adding to your collection, and when you are finished, you can ship them all at once.
for example i have been buying cards here and there to finish sets for the last month or so. and will continue to do so now ( as cards get added that i need) until i finish my sets and have enough to send to me to make it worth while. As i buy them they are added to my file and kept in storage until im ready to ship.
there is a great FAQ section, i read thru that heaps before i started using it.
http://www.checkoutmycards.com/FAQ.aspx
as for using as a seller i only know what is listed on the site and havent used it to sell ( YET) but i think it may be better and safer to sell there than ebay, but really want to know first what sort of cards sell more, as i mentioned in post above, it would be great to see ended/ sold items to get a feel for buyers trends on there.