Just look at the Community Market. With millions of competing users, if I wanted to sell a $10 game on I'd be lucky to get a couple of cents. I don't think sales and reselling are mutually exclusive, but from a purely financial standpoint, I'll make a greater saving on the latter by a significant margin.
There is the argument that I wouldn't buy so many games if they weren't so cheap, but that's for another thread.
Relevant.