Legal Question in Real Estate Law in Pennsylvania

The whole point of the buyer giving you a deposit was to put the property on hold. So no, you cannot sell to the other party unless you want to invite a lawsuit. Anything with land has to be in writing but if you start accepting other offers then you are going to be asking for trouble in the form of a lawsuit. Just don't do it.

You should have a written offer to purchase contract from the buyer outlining all the terms. Did you get one?

You are starting off badly here. Please get a real estate attorney - the buyer should be signing written offer to purchase contracts. The purchase should state what is being sold, for how much, when closing will occur, if there are any inspections or other contingencies, contain any required disclosures (like for lead paint or environmental disclosures), and indicate what is going to happen with the earnest money if either party breaches the contract.