Are you talking about seller financing your down payment to a mortgage?
A normal seller financing would be the seller holds a promissory note from you that after you pay the seller the downpayment (whatever the amount you and the seller can agree on), you will pay the remaining balance over a period of time for an interest (same thing, whatever you guys agree on). Basically the seller is acting like a bank.
Usually your Frannie Mae, Freddie Mac loans are much better than a seller financing deal. You would only go to exploit the seller financing route if either you can't get a conventional loan or for some reason you convinced the owner of the property to give you a better deal than conventional loan. It's very possible, if you are lucky, charming, or whatever found a kind person who's allow you to do that.
From the owner's point, there is definitely higher risk from doing seller financing. So usually the owner might ask for higher interest than the market rate. Or the owner does not want to take a lump sum of cash but rather take an installment over a period of time (taxes, retirement, trust fund, could be any reason).
Or, you could also convince the owner to take a second position after the bank. This creates even higher risk to the owner. Say if you'd like to purchase a house for $2Mil and you can only get $1Mil through the bank. You could ask the owner to seller finance the other $1Mil as a second mortgage. However, if you default on the bank's note and the bank foreclose on you, the bank will take the property away and the owner left with nothing.
There are property owners out there who want to do seller finance to collect the initial down payment and then just hope the borrower default so they can foreclose on the borrower, take the property back and repeat. So these people make money on the down payment.
All in all, seller financing is very flexible. Basically just comes down to what kind of agreement you can enter with the seller. As long as both of you are adults, with sound minds, not under durress, and there are nothing illegal act in the agreement, you can enter into any agreement to help you purchase the house. Just need to find a lawyer to do the contract.