What you're saying is correct but also puts a non-sophisticated seller in a poor negotiating position. They do not know what existing data the buyer uses or how their data can be complementary to anything.
I mean, I guess that's something that you can do but it's likely to get you laughed out of the room and greatly encourage them to reverse engineer whatever you were trying to sell.
You can always come back around, but if they pursue you, that tells you something about their appetite.