No matter how you decide to do your sale - always carry out an independent property valuation first. This helps you to estimate how much your house is worth and where to set the asking price. Chartered Surveyors not Estate Agents carry out independent valuations. Estate Agents provide a marketing service for your property; they don't provide underwritten valuations. It is a common misconception that Estate Agents value your property for you; they simply give you a very intuitive estimate of what they think your house is worth. Moreover, their estimates might be influenced by their motivations; they might set the asking price too high to secure your instruction or too low to obtain a quick sale.
It is perfectly possible for you to perform your own estimate of your house's market value by comparing the prices that similar properties in similar locations have been sold for recently. For further information, have a look at the Property Valuation for Home Sellers guide.