You can see what they're for here: https://www.qsa-global.com/880-series-gamma-ray-source-projectors
Yeah, the craigslist ad is more than a year old: https://archive.ph/Zpxp2
Oh, it looks like someone stole a truck that had some medical equipment on it. That is a bit troubling, but would maybe at best be enough for a small dirty bomb.
This used to happen in Russia after the USSR fell all the time, people would raid hospitals and steal the radioactive isotopes out of the old machines. I once read that every single person who stole it wound up dying because they didn't know how to handle the stuff.
There's a book I read all about it that is just fascinating. I'll dig it out of my library and add the title here when I find it, it's really worth a read!
I don't think you have a firm grasp on this.