One of the reasons we campaigned Admin to create multiple collections was because some of us collect multiple copies of cards. For example, I collect complete sets, Phillies, and HOFers. (HOFers that are Phillies only exist in the Phillies collection.)
Say I'm trying to put together a set (Complete Sets - Baseball collection) and I'm trying to get the Phillies cards (Philadelphia Phillies collection) as well. I have those cards in my want lists twice. Now, I make a deal with someone to get 1 of each of the Phillies cards. Which collection should they be added to? Phillies? Sets? Each person could have their priorities set up differently. Therefore, when Admin implemented the multiple collection feature, he decided that all cards transferred in a trade/sale would go into the "Main Collection" (the first collection in the list, which can be renamed). Those if us who have decided to use multiple collections largely have emnptied the "Main" and use it as a transitional point. I like to look through the cards I have received and move them in to the respective collections I have setup.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).