It's easy to do, just follow the Walmart (or other large corporation) business model: Maintain artificially low prices long enough to destroy the rest of the local market, create captive customers, raise prices.

In the case of a seed supply company, this could be done either via buying up competing farms and selling the end product for much less than your competitors; alternatively, supplying your seeds so cheaply that farmers would be stupid to use the alternative. Once the traditional seed supplies are gone, enjoy your monopoly.