A sale can't happen until the buyer and seller agree on a price. However, there isn't just one price in a real estate transaction, despite what websites like Zillow want you to believe. (Though Zillow does admit that their one "zestimate" number for a property is "not an appraisal", and they encourage people "get a Comparative Market Analysis (CMA) from a real estate agent".)
To get the best value, work with a licensed, experienced agent who is skilled at negotiating and knowledgeable about the avalanche of prices and paperwork that can pop up, from fees to requests-to-remedy to the pile of documents you will see at a closing.