[mothered]

But would they even check that? I don't think they write down the serial number of whatever monitor they send out

They do for verification/Identification purposes, as well as Inventory record keeping.

so if they get one back and it's the same model (just not so new anymore) it shouldn't really matter, at least that's what I thought

Don't assume and particularly, underestimate Amazon.

As of late, they're very thorough with checking returns. Cover every angle to help ensure a favorable outcome.