Psycho Ballistics cockers aren't very good.
http://www.pbreview.com/products/reviews/2400/
Be sure to read the lowest ratings as they're the ones that will show the true side of any product.
It's easy to say "OMG this is great buy it! 10 10 10 10 10!"
But it's harder to say "Well, the 3-way leaked when I first took it out of the box and I had to replace it but then my ram broke and so I had to buy a new one" Stuff like that.
They only average about 240 bucks new by the way, and I dont know what the guy is trying to sell it to you for.
Don't buy Psycho Ballistics cockers. Get a WGP
BUT, if like many people, you have your heart set on this and won't listen to anyone, then you MUST get your tank anti-syphoned. And on a cocker that is prone to leaking already, running any type of co2 through it is a bad idea since the leaking could be caused by crappy o-rings, and co2 would make them even worse.