Tehy won't be able to, thats the problem. I can remember renting computer games from a store, back when 7th Guest and Descent were just new, but thats before piracy had become such a big issue.
The only way it could ever happen is if a rental store, say Blockbuster, for example, paid Valve, for example, for 200 CD-Keys. Then, they charge you $30 or more to rent the game, and give you a piece of paper with the CD Key on it, al-la HL2 voucher from ATI, and its your CD Key to keep, you just return the game. Valve makes money, Bluckbuster makes money, and you didn't steal the game for nothing.
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