What do I need to watch out for when buying an unlocked phone on the used market?

I want my next phone to be "unlocked" so I can use it w/ any carrier - Verizon, AT&T, T-Mobile, Mint, etc. I'm based in the US, but if I travel internationally, it would be nice to be able to just pop in a local SIM card and continue using my phone like normal.

What gives me pause is, I see used phones listed as "US Version," and reviewers claiming the phone was not unlocked (for example, this Pixel 7a on Amazon).

Questions:

1. What am I missing here? Do these reviewers just not know what they're doing, or are they buying from shady resellers?
2. Does "US Version" imply I would not be able to use the phone internationally by just popping in a local SIM card?

Thank you!