a person going to buy a used glock will not give a darn about the barrel not being original
As someone that has bought used Glocks, honestly I would give a darn. No it's not a collector item. But I'm going to wonder why the original barrel was replaced. Was it a squib load that went unnoticed? Was it shot out from use? Either of those will make me look a lot harder at the rest of the pistol and check for excessive wear, and I might hesitate a bit more on the purchase as a whole.
That said, keep in mind this is a supposedly new pistol. If you bought a supposedly new car and found out after the fact that it had a replacement engine, wouldn't you wonder why?