Yep, some risk and reward. Batteries are so pricey that people buy larger sets and remove the batteries and a tool or 2 and then sell off the rest. Seems like 6 of one, half dozen of another.
Once they hassle with selling off the tools they don't need, they probably could've bought a 2 pack of batteries. Not sure if that's true in all brands. In the Ryobi lineup, it is.
I usually just keep the spare tools as backups or barter with them. I had a spare Ryobi cordless impact. Gave it along with a battery, charger, and a Ryobi bag to a guy in exchange for about $300 worth of concrete work that he helped me with. He was so happy to get a much needed tool without any cash outlay on his part.