Also, you have to consider that people like me who use both the trading and deckbuilding aspects of the website.
I have my Avacyn listed as available for trade, when in reality you'd have to kill me for her lol.
The reason for that is because it's too much of a pain in the ass to list your cards available for trade one at a time if you'd like to exclude the ones you have more attachment to. It's far easier to deny a trade every now and then, versus having to select each card individually and add/subtract it from your trade and wishlist.
I have my entire collection listed for record keeping as well as trading. I like to have one copy of every card from a set. If I don't want people to ask for my last copy but still want to trade with people, should I have to go through my entire collection set by set, block by block and remove all the cards with only one listed.
Even thinking about that gives me a headache.