Classifieds are restricted until you get age verified, proving that you are over the age of 18 and can buy/ sell airsoft guns legally.
It also helps because it means that two people involved in a transaction can be near 100% certain that it's a two party exchange and there's no awkward parental involvement.
It may seem like a bit of a hassle to get verified initially, but the system works. It's also helpful when a deal goes south, because while the site itself has no responsibility to either party in a private transaction, at least there's a paper trail left behing that can get a scammer to be known publicly.
Check the stickies to find an age verifier in your region, go have a coffee with them or meet them at a game, and it'll all come together.