It would be best to ask the agent for the Vendor's reason for sale. Do you believe the property is worth more or less than the asking price?
Zoopla will help you to produce an estimate for the property. Otherwise standard due dilligence is best including a mortgage valuation, homebuyers report or structural survey to ensure the property is in good condition. Also consider external factors such as neighboring properties and future developments in the immediate area.
Also, do not discount the fact that the vendor might need a quick sale for whatever reason.