Wrong as in incorrect, or wrong as in morraly not right?
The EULA that you get with software specifically says you're not allowed to sell on the software, in fact, you never own the software or the product, you only purchase a right in which to use the software. So technically, you can't sell you're rights as it's an agreement between you and the vendor.
However, I'm not saying this happens now, and 99.9% of publishers overlook this, but if the games were widely available to download, then I doubt you'd be able to sell it on as they'd make a loss.
As for Valve, well some of their products are cheaper. Darwinia is cheaper to buy through Steam than it was retail.