What items can be sold on Steam Marketplace?
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.
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
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 :)
Are these items that people have found in game and then are able to sell on the marketplace or are these items that they have bought with real money and are now trying to resell for real money?
It is a combination of both. It depends depends on the game. For TF2 the items can be found, awarded as a pre-order gift, or received from a crate (they key either traded for or bought from the store)
can I expect to make steam money off of items which drop in game?
Do not expect to make much steam money. The items that are common do not sell for much. Only items that are extraordinary rare/hard to get sell for more than pennies.