Think of it from the buyer's perspective
Item A is a brand new Glock, which if anything goes wrong, the store is going to address the problems, or Glock itself will. Price $500
Item B is what appears to be a lightly used Glock, sold to you by a total stranger who may or may not have tinkered around inside, may or may not have abused the gun, may or may not have legally purchased the gun in the first place, etc etc for $450
In my experience most people will pony up the extra $50 to get new.
Think of it more as a $550 new and $400 used and you have a better ballpark. Note, a buyer would expect to pay $400...at a gunshow or add in the paper. If you sell the gun to a gunshop, as they are really middlemen, they are going to want a cut and will probably only offer you $300 for it.
The other thing to realize is that money inflates. By that I mean when Grandpa was a kid, a bottle of Coca-Cola cost him a nickle. I remember when a can of Coke would cost 50 cents in all the vending machines, now 75 cents or 80 cents is the going price.
Well, luckily for you, guns tend to go up in value right along with inflation, and few other items do. A glock bought in 1985 was probably bought for $300, and lightly used would probably sell for $400...just like a new one bought a year ago woud. However, a TV or a VCR or a cordless phone or a stereo or a computer or whatever bought for $300 in 1985 and lightly used, what would you be able to sell that for? maybe $10 at a garage sale?
This makes guns a reasonable long term investment. Not a super great investment for you to retire on, but after you take into account the initial price drop between new and slightly used, the gun will hold it's value over time.