Once you've consumed the CD key from a retail title by associating it with a Steam account, you can't use it again.

Furthermore, Steam will not allow you to merge accounts or move keys between accounts:

Steam accounts can not be merged together.

Per the Steam Subscriber Agreement, Steam game subscriptions / CD keys are nontransferable and cannot be reset / moved between Steam accounts.

The only exception that I've ever found is Company of Heroes, where I've been able to give away my account keys to other people, even those purchased on Steam. They limit you to 3 transfers, however.

The Market is still under development. More items are likely to be added later.

One way to put an item up for sale is by going to the Marketplace and clicking "Sell an item".

This will show you the eligible items for each game (and steam, overall). change the game filter by clicking on the game you want to see items for.

You choose the amount you want for it, then Steam will add a bit (as well as the game in some cases).

There are rules, however.

1. Items purchased from the TF2 or Dota 2 Store may take up to seven days before they are eligible for trading or sale in the Community Market. Depending on how recently you began making purchases, these items may not initially appear eligible

2. You need a purchase through Steam on the account older than 30 days (without any one-year periods of inactivity which would reset your eligibility). This purchase could be for wallet credit alone. A CD key activation is not sufficient.

It is possible to make a little money off the Marketplace. I sold a few trading cards and had $2.50 to buy System Shock 2 with :)