1. Location - Not everyone lives in an area where gun shops are on every corner. Hell...some people don't even have a gun shop within 2 hours. When I lived in Alaska, I lived on an island that had two gun shops that barely had anything good in stock or even at a reasonable price. But lots of people had FFLs.
2. Buying out of state, regardless of dealer or private individual, most of the time you will not pay sales tax. Sure, ya pay $15-20 in shipping, and depending on your dealer, a $20 transfer fee, but in many cases thats still cheaper than sales tax.
3. Selling local in person, you reach very limited number of people. By listing online, you can reach hundreds or even thousands daily. Depending on what you've done to your firearm, you may have to find a person who actually wants exactly what you've done to your Glock in order to get your asking price. Or if you're desperate to move it, you can get more offers quicker.
4. Not everyone knows where to look for used firearms and/or police trade-ins...and if they do, maybe they want to buy new anyways. Some people aren't comfortable with buying what could have been your problem only to have it become their new problem.
5. Not everyone knows about discounts available to them whether they are LEO/Military/EMT/Fire/etc, or even a GSSF discount on new Glock purchases.
6. Don't forget supply and demand. Every day people complain they are put on a long waiting for the gun they want. Some people will pay even more just so they don't have to wait.
Used Glocks are like used cars...most people are willing to budge on the price if you make a reasonable offer. If you low ball them then you're likely to just piss them off. If you can purchase a used Glock from a dealer or pawn shop for $350 or so...don't bother wasting a private seller's time as most aren't going to go that low just cause you can get it near you for that price.
Too many variables to sit here and complain about people pricing their firearms for sale that you obviously have no intention of buying. Just my opinions. Have a good night.