Why it's a good idea to have a tradelist:

You can post it up in the forums so that traders can look at it and know what you have for trade...

You have something specific to show others when they ask you "what do you have to trade?"

You won't appear to be a lazy trader. (These are the people who post: "I'm looking for these specific players, I have lots to trade") Lots of what? If you can take the time to post your wants, take the time to organize your tradelist!!

In addition to the obvious points above:
You have a written record of all the cards you have in case they get stolen or visited by some natural disaster.

So you have too many cards to list?...have them listed to generate some trades...not everyone is after only GU/autos of the rookies who might hit NHL ice this season.