There are a number of ways to sell and promote games these days.
One approach is based on scarcity, making provision of goods and services to individuals who pay for exclusive access to them. Purchasing is usually dependent on established prior relationship/investment and is dependent on reputation. Examples of this include subscription models like Dungeon-A-Day, D&D Insider and World of Warcraft. You pay for access to use services unavailable to others and you get some tangible benefits for doing so. Pays your money, takes your choice. A twist on this is the ransom model where X number of people contribute to paying for the publishing of a book who get the book first.
Another is based on prosperity. The more people have it, the more play and talk about it. And if it's good enough, then more people play and talk about it and the cycle spreads. Such viral-based approaches have seen an increasing rise given the presence of the Internet but have been around for some time. The concept and some permutations are well-documented in Seth Godin's 'Ideavirus'.
The Grateful Dead were original exponents according to this Atlantic article of the Connect With Fans + Return To Buy model used by bands like Nine Inch Nails, Coldplay and Jonathan Coulton to sell their art to us - a model being replicated in the games industry. Games like Swords and Wizardry, Mutant Future and Eclipse Phase have freely-available electronic copies and all have grass-roots support.
A third road is to operate a patron model. This can be combined with crowdsourcing - Open Design are doing wonderful things with books like The Red Eye of Azathoth and Tales of the Old Margreve. Once the book is published, you can buy it and if you've got in at the ground floor will be given credit where credit is due. Another exponent of this is Nevermet Press, whose first product The Desire was received by everyone who participated in DriveThruRPG's Haiti donation.
Now imagine if playing the game gave you rewards just for playing. You could start with intangible award badges (say 'Dragon Fragger') to display on your blog, Facebook page or games console. Then escalate it further - offer discounts on products or even gift vouchers for retailers with the right connections. And how many online retailers are going to sniff at a loyal tribe of potential customers? Which method or combination would you choose, as a designer or more importantly, as a consumer?
(inspired by JoeTheLawyer, James Edward Raggi III and Jesse Schell (via Justin Achilli))