You can't always require county, as mentioned above, this can end up being huge.
Breaking it up into the zones above won't work, because one person may be on the border and may easily be able to drive to another zone, and another person maybe in the west of the zone and the other in the east, which is too far to drive.
City would be better, but living in socal myself, city doesn't do much because I might drive through ten different cities to go to a PPT.
->The BEST solution, would just be a requirement to give some sort of indicator on location in the title, with this indicator being suited toward that person's location. Location indicators for socal won't work the same way as they do somewhere else.
If you are in Bakersifeld, say "Bakersfield"
If you are in socal, say "LA/OC", or "OC/IE"
If you in east bay, say "east bay."
People in different regions will [should] know how to specify their particular region within a reasonable driving distance so other locals will understand.
Now, that said, I am generally opposed to breaking out into different forums, as I'd drive quite a distance for the right thing.
WTB: HK21A1 & HK13E parts
WTB: Project & fixer upper guns: Old beaters needing some TLC, C&Rs, Mauser's, Colt's, Beretta's, & HK's