In order for an offer to be legally binding, the offer for the purchase of a property should be made on a formal contract document, irrespective of whether the offer is via a real estate agent or directly with a seller (the vendor).
A verbal offer is not legally binding and should not be relied upon by either party.
When an offer is submitted to the vendor, they have several options. A vendor can agree to the offer price and terms and sign the contract. If the offer is not acceptable to the vendor, the vendor can amend the price and/or terms and return it to the buyer for further consideration. This is known as a counter offer. This back and forth process may continue until both parties are happy and agree to the terms of the sale.
Where both parties cannot reach agreement on the offer price and/or terms, the parties will walk away from the deal.