Owner financing is often a fantastic deal---for the owner. For you, the buyer, not so much.
As your question suggests, it's relatively rare in New York City. It's more common in areas where there are properties that are difficult to sell.
If you can get a loan through a conventional lending source, you will almost certainly be better off. If the owner won't accept such a loan, find out why.
Here in Brooklyn NY it's entirely possible there is something about the property itself that makes it unbankable. You need to find that out, too, if you want to buy this place. The possibilities are too numerous to go into here.
You really need to hire an attorney who is an experienced real estate specialist to negotiate any owner financing agreement. This is not a job for your lawyer cousin who does estate planning all day.
Marcia in Florida is right, there are investors for whom owner financing is a major component of their business model. But we have a different business culture here in Brooklyn NY than in Florida, and a competent attorney really ought to flag the pitfalls for you.
Licensed Real Estate Salesperson
Charles Rutenberg LLC