In my experience, many owners that offer seller financing want to offer a second mortgage to help with the downpayment, and ensure that financing is secured. Does the seller own the condo, or is there a mortgage attached? Is the seller in a distressed situation where he/she needs the money immediately?
From the seller point of view, no...no skin in the game. What interest rate/amortization are those payments/pay off based on, or are they just random numbers?