tax assessment versus appraisal

[Arcinio Arauz]

I found an old small house and asked the owner if he wanted to sell as is. He said yes! How much? 35K...I said ok give me a few day and we'll sign a contract and go to title. It needs a full rehab and has an ARV of 97K according to online comparisons and a few homes with same specs near by.
So I looked at the tax assessment and was a little confused because the assessor valued it at only $9,500. So how can I get a better idea of it's value with out having to hire an appraiser? What's the difference between the tax value and on online estimate value such as (Zillow, Eappraisal, Red Fin...ect)