The Smart Ones.

If you want profit, i.e. trading an older game for Skyrim, find what people like, and asses the availability.

Consider things such as:

Demand - Sales Ranking (check the top sellers tab on steam)

Availability - Check if the game has been in bundles or special promos.

Customer's Feeling of Want - How much they are willing to be low balled on a game they are desperate for.

Reviews - If IGN says the game sucks people wont want it.

Valve Games - Everyone has all of them accept for the new betas.

There are other stuffs that need adding but this is just a prelude.