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Computers tend to have low markups. The market is very competitive and the only real difference to casual shoppers is price. If you sell a Windows based system you get to offer antivirus and office software which has a better markup and then installation and setup. A linux box comes with most of your software needs available for free through their repositories. You may still be able to get a setup sale but that's about it.
The only way you can get shelf space in a chain is if you can provide more profit for the retailer than with a Windows computer and I can't see that happening