At the risk of raining on your parade, you've done a very silly thing. No responsible dog owner would sell their dog on Facebook.
The people who do this tend to be the kind of people who have not bothered to train the dog and it's become too much for them to handle or they've just found out it has x, y and z health problems and they can't afford the vets bills.
They'll also lie through their back teeth to get rid of the dog for what they think is a fair price and recoup some of their losses. I'd be right if I guessed they either have to move or have just taken on more hours and don't have time for the dog wouldn't I? Bollocks, they have. They haven't bothered to train their dog and now they can't handle so they're passing the buck to someone else 
In your favour you went for a Staffy, which, luckily for you, are fairly healthy breeds and want nothing more than to please you, they're also excellent with children and are nicknamed the nanny dog because of their natural love for children.
Get the dog insured now as in right now. Vets tomorrow for fleas, worms and a full health check and a microchip. You can discuss nuetering or spaying while you are there.
Book into a training class this week. The decent ones round here have a waiting list. Devil Dog has been on the waiting list for obedience classes with the only positive trainer in our town since early last year. I got an email yesterday to say if I can pay the fee now there is a cancellation for August's class otherwise he is in October's group, almost a full 12 months after I applied.
If this is the case in your area I would strongly advise that you pay out extra and do some one to one sessions before any problems become noticable.
Good luck OP. I truly hope it works out for you, not least because I adore staffies and it breaks my heart to hear about them being passed from home to home because no one has bothered giving them the training they need. They can be boisterous, but they love to learn, simply to please you, all they need is someone willing to put in the effort.