If you have 100 copies of professional looking product, I think you have a decent chance of driving around to 20 stores and getting 5 copies on their shelves, each. It will take a little wheeling and dealing, negotiating on what the retailer's cut is, and when you will come back to collect unsold copies. It would behoove you to be well dressed and groomed, smiling, with a gracious presentation. Doing a little research with some retailers ahead of time to give you reasonable expectations would be helpful as well.
There are a lot of boxes to check in the previous paragraph, which is strangely difficult for many people to perform.
Exactly.
In the end, shelf space is the most important thing that any brick-and-mortar retailer owns, and some shelf space is particularly valuable (end caps, for example). Any item that sits on a shelf is taking up space that another item could occupy, so whatever is most likely to sell - quickly and/or on a continuous basis - is going to be the primary focus. It can be hard to justify occupying that space with products that sell slowly. Products that don't move steadily also tie up money, which is even more of an issue for small businesses operating on slim margins.
Removing those risks makes it more likely that a new product will get placed in a store. Collecting unsold copies, or even doing something similar to consignment selling, helps retailers avoid the risks associated with investing their money in something that may otherwise sit on their shelves for a long time, steadily decreasing in value (since it is taking away from other potential sales during that time period). A professional appearance, demeanor, and presentation can go a long way towards gaining the trust of a business owner, as well.
Offering to run in-store game sessions where you teach people the game may be a selling point, too, with some retailers.
Having said that, placing an rpg in some individual stores might not do much to spur the sales or popularity of a system. In many cases, spending that time focusing on online advertising and promotion may be more effective.