if Seller doesn't need your seller financing deal to sell the property then you probably should move on. You can't add any value to his deal. Maybe consider overpaying for the benefit of having the seller finance. If i had a choice of seller financing my properties or full cash out, the answer is simple, take the money and deploy it elsewhere.
Seller Financing is the ultimate way to purchase a property! I have purchased this way and I can understand your aim to do so. Getting a seller to except seller financing is extremely rare and there is no way to know what sellers would be open to this. Are you focusing only on doing seller finance deals or are you trying to close by other means if you find what you are looking for. I always make 2 offers to a seller when I am making them an offer, the first would be a lower cash price to buy the house as-is and close quickly, the second is a "Terms" offer and will be for more money. This will give them the option to except a seller finance offer if it makes sense for them. Again getting seller financing is tough but not impossible, once you have done them it will give you more credibility to the seller.
In order to get a seller to except seller financing they first have to be open to this, many people are using the money from the sale to make their next move so its not an option for them. When you find yourself talking to a seller who has a free and clear house and seems to not be "needy" then explain to them why this is a better option. "We can turn the house into a stream of monthly payments that will last for years!" Find out what they want.. They will get much more money for the house plus can save a good bit in taxes they would pay on an outright sale. It has to be the right person and the right property, work on learning and explaining the benefits.
As noted, if the guy can sell to you for market doing seller financing, what is the benefit opposed to FHA or Conventional sale to him - those funds he gets up front and the bank worries if the buyer will pay, not him. You don't bring any value to the table and put all the risk on him.
From the flip perspective, I often will offer seller financing if I am selling to a non-corporate flipper or home hacker. I will do it with a partial down payment (to cover my costs) and 0% at a markup of what I would take cash (amount depends on the term). That way they don't have to dump all of their money into the property up front, and I get the equivalent of rental income for 3-5-10 years without needing to worry about tenants or fixing the property. My lawyer makes sure I am listed as a lien holder and I have a do-on-sale clause so I can call the note if they try to sell it. As long as they have a decent credit score/proof of income and I don't need the cash immediately (i.e. we are talking lower end homes, I am not doing it on a McMansion) then it has been nice on the couple deals I have done that way to line up income to cover bills for a few years. Hopefully I never need to reclaim a house - I am sure once going through that hassle two or three times I would scrap the idea.