Use a combination of ads, signs and direct pieces with the message "I Buy Houses" and your phone number and website. Do this consistently for a month and you'll get calls. You may also want to invest in a good program for marketing to sellers. That way you'll have a step by step guide with templates and a plan. BTW if your county only has 20k people in it that really is a problem as the numbers aren't there for your marketing to be effective; you may be able to buy, but selling will be a big problem. How far away is the nearest population center (city) with at least 50k people? You may want to shift your marketing to that area.