I hate that you're getting out of the hobby! It's always sad to see someone give up.
First, I believe you should list all of your cards. I only saw around 50,000...and none of them were listed as "For Sale/Trade"! If you get your cards listed, and put them for sale/trade, I know that folks will go through and offer to buy many, many of them!
To update your profile, click on your user name in the upper right hand corner. You'll see "Profile". Click on that. When your profile comes up, you'll see an option in the upper right to "Edit Profile". Click that. Then change what you want to change. Once you are done, scroll to the bottom of the page and click "Save Changes".
Trades are turned off for now.
“If you find it hard to laugh at yourself, I would be happy to do it for you.” ― Groucho Marx