While seller financing is a viable option for purchasing a business, it is important to understand that a seller is looking to assess the risk level of their investment (you) and their proposed rate of return (purchase price, amount financed and proposed interest rate) before extending an offer to finance the deal.
The keys to obtaining seller financing are: demonstrating competence in the business and industry in which you are investing, proving your financial ability, and offering the seller an attractive rate of return. If you approach the situation understanding that you have something to prove and are prepared to demonstrate the competence of your management team, your understanding of the industry, and your financial ability, most sellers will consider you for some level of financing.