Not a lawyer, but my guess would be that whether it is legal or not depends on what (if anything) you intend to do with the information. This is based mainly on perusal of this page summarizing a bunch of state identity theft statutes:
https://www.ncsl.org/research/financial-services-and-commerc...Of the ones with details listed it seems most of them require intent to defraud or something similar in addition to possession of personal information. I think paying for the info could also be problematic, there are a few trafficking laws in that list.
In a similar vein, possession of the info with the intent to use it to hack into something would probably run afoul of the CFAA or other anti-hacking laws.
If you collect that info with the intent to submit it to haveibeenpwned I think you would probably be fine.
If you collect that info just for fun, but don't do anything with it, I suspect that's legal, but probably not well-advised, as I suspect cops/prosecutors/jurors would have trouble believing someone did that for fun, and would interpret it as evidence that you were up to something nefarious.
I don't think the stolen property angle is an issue. Digital information can't really be stolen: usually the problem is you've committed a copyright violation. In the US at least I doubt user names and passwords would qualify for copyright protection. (Generally, collections of facts do not qualify.)