Your questions are answered right now. $400 doesn't buy that whippy of a bike. It's okay for mild entry level riding around. Full suspension under a grand are really junk. Under $500 I'd run away. Buying used is a smart bet for folks on a budget. Sometimes people with more money than most purchase a good bike and then decide to not ride. They sell for much less then what they paid.
I get the impression you want to hear it's okay to buy a "full suspension" for $400. Well it is if you don't really want to venture off the pavement. If you want to ride the dirt with any regularity it's not okay. It won't last long. $400 bikes are are pretty much the same. Fit will be your main difference. If you begin to ride seriously you wil quickly outgrow a $400 bike. Sell a gun or a fish, sell off some extra shiny chrome parts off the truck. Or just go ride some entry level bikes until you find one that fits the best.