In my experience (2G, 3G, 4S, 5), the glass holds up really well. Any small scratches that do occur aren't noticeable when the screen is on.
As far as resell value, I've never gotten less money for having a phone with minor scratches on the screen. I list it as near mint, and no one has ever complained when they saw it in person.
You have had some good luck, I wouldn't consider an iPhone with scratches near mint, maybe good condition depending on the amount and size of scratches along with asking price of the phone.
Of course in my area, there is alot of dust and dirt blowing around which causes scratches almost instantly.
I have used the ones from Verizon and they seemed to hold up well.