I have just recently found this site and created a list of the cards I currently have. I'm just starting to get back into MtG after quitting playing in 2001. Entering the cards you have can be a bit time consuming...depending on how much info you want to supply. Using the mass up-loader is actually pretty easy. All you need is a card list and the quantity you have. I happened to have my card list in excel format which made it much easier. I just copied and pasted the list into the mass up-loader. You can also have it enter the quantity for each card when uploading. All you have to do is copy and paste the list then go back and enter the quantity for each card before the card name. Be sure to put a space between the card name and quantity. You want it to look like this:
3 Serra Angel
5 Boomerang
7 Dark Ritual
Once you have the list and quantity slotted. click the import button.
After that...you will have to manually enter the quantity of cards you have available for trade and any other info you want to show (Ie...which set, foil, promo. textless and condition).
It can be a bit time consuming but at least you have a complete list of your cards. One thing I hope they implement is the ability to sort by set.
Great job so far to the developers.