Purchased item at asking price on Facebook marketplace, she agreed gave me address, then sold it at a higher price. Legal binding contract?

There are many other factors which would determine whether you had an enforceable contract, but there's no reason that an agreemeent made over Facebook couldn't be a binding contract.  Assuming that you had a valid contract, you would have a claim against the seller for the difference between the price you agreed to pay and the price you had to pay to replace the items you had contracted  to buy.