Seller financing is when the person selling the laundromat will take payment terms on the laundromat they are selling. So if you are buying a laundromat for $100,000, and the seller will provide you with a loan for 10% of the purchase price, you will have a loan for $10,000 with principal payments and interest being whatever terms you agree on.
When I bought my first laundromat I had seller financing for $25,000 with interest only payments for 5 years and a balloon payment for the balance which was $25,000. The last two years of the loan I made larger principal payments to make sure it was paid off at the end of the term.
I also cashed in my 401k, put money on my credit cards, raised private funds and tapped the line of credit on my house to raise the entire $220,000 needed to buy that laundromat.
Does that clear it up?