The 9th U.S. Circuit Court of Appeals has ruled that software publishers can legally prohibit the resale of used software by including a clause in the software license agreement. I think this ruling is being appealed, but if it is upheld, it would mean used game sales would become a thing of the past. No one - including both stores and individual consumers - would legally be able to sell any games with such a clause. Any used game sales would have to be limited to games that were made before these clauses became common place.
You can read some more details and reactions in this news article.