Can I ethically and legally buy a used phone whose IMEI says "This device has been replaced, so it can not be serviced by Apple"?

That message means someone requested service on their iPhone, got the replacement (whether by Express Replacement or not) and instead of sending back the original iPhone, which was not broken, they sold it. It is a common scam. It is technically owned by Apple which can make it stolen property depending on the jurisdiction. And Apple absolutely will not service it.

I would suggest taking a hard pass.

Not necessarily stolen. The thing I can think of is the Express Replacement program where Apple sends out a replacement device with a credit card hold, and then charges that hold amount if the replaced device isn't returned in time. I don't know about the ethics of that other than I wouldn't consider it "stolen" unless it was somehow diverted.

Most replacements are done in person, so this would be unusual.

Note too that sites like that IMEI pro site are notoriously useless in reality. They claim to check devices world wide, but there literally is no way for them to actually acquire the data they’d need to even do that.

In the USA the only reliable stolen device checker is the US wireless trade association -> https://stolenphonechecker.org/spc/ and even it states it cannot be taken as a guarantee a device is not stolen. And it only represents data from USA carriers, not global carriers.

Note too that a common scam is to not report a phone stolen to the carrier until after it is sold.