I also live in Indiana. I have also performed numerous private Firearm sales buying/selling/trading. I also have to assume what you are asking, you are asking simply out of being naiive regarding firearm sales.
What you are asking is ridiculous. The vast majority of property in this country, firearms included, are not stolen and will never be stolen property. If you bought a vintage jacket at a second-hand store, I have to wonder if you would go out of your way to attempt to contact the original seller to find out whether or not the article of clothing in question had ever been stolen. The consequences are essentially the same. But there is no National Stolen Clothing Registry, even considering there are several articles of clothing in this country that cost exorbatantly more money than your standard service firearm. The thought of such a Registry is ridiculous, is it not?
After you opened a Birthday present at your last birthday event, did you make sure to ask the gift-giver if the item was stolen as well, or at least insist on seeing proof of purchase?
There isn't any reason in the world (or at least the State of Indiana) that you should treat this any differently. I create a Bill of Sale, or receipt, with my and the other party's information on it for every transaction. That is it. Assume property is not stolen property. I guarantee you will be right with better than House Odds. And in the case that you're not right, your liability and responsibility for the property will be covered with your bill of sale. And you will be able to aid the police in their property investigation by pointing them in the right direction.
But as far as requesting a lower price, or requesting the man submit his gun to a database test that puts him at additional risk of even a "False Positive", is ridiculous and incredibly one-sided. It is not something that will be received well in this community, and likely something that will be seen as nothing short of an insult by the majority.
If you showed me your nice new shiny car that you had just worked your butt off to pay for, my first response would not be to ask you if you had considered that you may be in possession of stolen property unknowingly.
I also realize it might upset you to read some of these responses, but I assure you that the dissonance you are feeling is simply a lack of understanding. This is not something that is commonly done, and for a reason. It is not needed, and largely is a power that can be used to take guns out of the hands of law-abiding citizens with regulations like "False Positive/Partial Match SOP". And possession of our guns is something this community takes very seriously and very close to heart.
~LT