Two thoughts come to mind.....first, if you are working with a realtor, have them contact the seller to introduce your interest in the property and see if they are willing to accept this arrangement.
Secondly, if this is not acceptable, introduce to the seller that any offer for the property must be supported by an appraisal. Banks require this and buyers paying cash for FSBO's would be well advised to do so as well.
Research. The problem is, just gathering the data doesn't mean you'll understand it or draw a proper conclusion. As someone who does this everyday along with other professionals, we often use the same source of information but come to some different conclusions based on our understanding and current market knowledge.
I know this isn't what you wanted to hear, but sites like Zillow, Eppraisal and others are very broad in their estimates and readily admit to be 8% to 15% off depending on the market.
It may make sense for you to hire a broker to provide some assistance in pricing, contracts and complying with the laws. I've assisted other FSBO clients for a reasonable fee. I'm sure others may be able and willing as well.