Tax assessments are tricky, as you have to make sure the town has been recently re-assessed, or else it may be inaccurate. Also, if significant work has been done to the property and the town doesn't have record, this may affect assessed value also. Appraisals are the same- when was it last done?
Also, a market analysis done by a realtor will help you answer this question. We look up sold comparables and what is active on the market and come up with a price range to offer accordingly. You just need to work with a buyer's agent and they can provide you with this service :)