Its not, you set up an arrangement with a local verifier (Which is as common as fat kids at a fast food resturant in your area)
You spend 5 minutes or more with the verifier, talk about the sport, educate yourself, and give your date of birth, name and username to the verifier.
A couple weeks later you get access to the classifieds. Simple as that. You're not paying for anything other than the gas required to drive there.
And when we say 150 isn't enough. We mean it. Anything below that price is generally really low quality and not worth the money in the long run. If you invest in a real airsoft gun you'll notice quite the improvement in quality and perfomance.
Keep in mind, a 'low cost' AEG runs anywhere from 250 - 500 dollars. Actual spring rifles go for around 350 - 500 to even +1000. And this is bone stock (Not including upgrades, which is more of an investment.)
If you're looking to plink, buy an airgun. Best advice we can give to you.