Er, you cannot directly trade CRS/CE for Steam Credit.

You can, however, trade CRS/CE for Trade-able Steam Items/Games. You simply decide a ratio (CE/$; CE Per Dollar) with a seller; then said seller gets the Games/Items (trade-able) that you want under said ratio.

For example; a trade-able steam game that costs 5 dollars at a 500ce/$ ratio would be 2500 Energy.

Most sellers go for a rate of 550CE/$ - 650CE/$; sometimes 650CE/$ - 750CE+/$ if they're aiming high.
Most buyers go for a rate of 500CE/$ - 600CE/$; sometimes try 450CE/$ - 500CE/$ if they're lucky. Most sellers would avoid/decline rates that they consider too low though.

Basic info; hope it helps

Raptor'z