Back in college I got my phones this way. Pretty much dependent on in person swaps where I could try to test things but I guess there are probably sites that streamline this these days.
And… I had no choice at the time. Couldn’t afford even the “cheap” phones. But… I would never do it again and I think I spent more money in the long run.
Because, with very few exceptions, people aren’t buying androids because they need the latest flagship phone. This isn’t an iphone where people think their human value is dependent on how old their iphone 95 is or whatever. People either (effectively) rent their phone from tmobile or they drive those into the ground until the battery is dead, the antenna is damaged, etc.
So I get a “good price” on something with a fairly short shelf life and just get frustrated when I actually use the phone. Which results in more frequent updates that I “could afford” because they were “so cheap”.
At the end of the day: If it is all you can afford and you need a phone now, it is a decent solution. But I will generally say it is better to try to save up for another few months/a year and get one of the cheaper phones that exist these days.
Because, checking the google fi site (which is usually a worse deal than having a “real” provider): you can get a last gen refresh 7a for 300 bucks, new, that will last you 2-4 years. Or you can get a moto g 5g for 30 USD (what the flying hell?).