You are a Realtor, use your Realtor thinking cap! You know how to figure out what the market price of a property is based on comps, right? Then figure out what the market terms are for owner financing. If you search on your MLS and check the owner financing options, you can oftentimes see what terms are most common for listed and sold properties. In my area, if I check that box, only a few listings pop up, but most of them say in the agent-only portion that the seller "will accept 30% down at 10% interest" or something like that.
Approach the seller with this data. It only takes 10 minutes for you, and you can tell the seller "Based on my hours of super extensive research, I've noticed that owner financing in this area tends to be 30% down at 10% interest for 15 year terms. Would you take 30% at 10% interest, but with a 20 year term?" and go from there.
Also, here is a pro tip from an attorney's standpoint that I've seen from my clients that have done stuff like this. They tell the seller they already have real estate attorney that will draft and file the loan documents (which in CA is a promissory note and deed of trust) for a small flat fee of X amount. I know buyers that use this as a negotiation tool for a seller financing purchase because the seller knows they won't have to deal with finding a lawyer themselves, and it's basically a done deal once they sign the necessary loan paperwork.
Generally speaking, this is part of the practice of real estate: make the seller think you have done this a million times already and you have the people and paperwork in place to get everything done inexpensively and easily. Make them think they don't have to do sh*t except sign over their property to you for a nice profit.