seller financing offer

[DJ Cummins]

So the people we bought our first property from last month own 5 other properties. They hinted to us at closing that they would be interested in possibly doing a seller financing deal on at least one more property. I have read some about seller financing but i dont really know how it works.
Say we were to take a two year seller financing option...that means in two years we would have to come up with money from a bank loan to refinance and pay off the balance with the sellers. Do we need to have 20% down to refinance a seller financing deal? I dont even know what other questions i have. Any advice would be appreciated.

thank you!