I have been gun buying for ~20 years. In the past, I thought new meant I was getting the best possible chance at a great running gun.
Now, this sounds weird, I know, but I would prefer to buy used, especially semi's. A worn, shot a lot semi says to me that a guy has shot it enough to know it was reliable for him/her and then carried. That is generally what I want in a gun! I can either deal with the finish wear or pony up for the refinish. An unfired used gun is a bit suspect, but can just be well cared for. You have to read the seller a bit.
I have only bought a few used, but I'll keep looking that way.
I've bought new, usually at a gun store. Several of them were fine, but one was a real nightmare that I turned into a 99% custom....the ejector MIM finally "broke" and I'm using that as a starting point to finish it's conversion to all tool steel internals! My point is that new can be great or can not be.
So, I feel like even though used has nobody backing it that there are ways to know how it runs.
Although as I say this, I bought my last gun new! Still, my FAVORITE CCW was a used Kahr I bought at Buds. Who would have thought, Buds, used, one pic would be ok. Call them, they are willing to take care of you on a used gun.
One thing to look out for or look for are used IPSC guns. They can be a great deal or a pile of issues. If you know the platform and know how to work on them, they may be a good deal. I think I would want to detail strip and function check every part before buying! "Hmm, do you mind if I look at this $2000 1911 for like 2 hours and shoot 100 rounds through it?" They might mind...
I bought a used LEO Glock 21 that looked well used in the pics! After a full cleaning, it only needs a front sight, as the tritium is a bit dim....price was really good from a reputable FFL!