DETERMINING THE FMV

Should the Tax assessed value be used to determine the FMV? Theres a few homes Ive looked at, and the asking price is way above the Tax assessed value. Also when I compare similar homes in the area using the tax assessed value, the FMV of the similar homes is far from the home im considering.
For example:

My mother in law is also considering on buying a home she is currently renting. The guy wants 80,000, she had it appraised at around 65,000. When looking on the countys web site for the tax assessed value, it has a FMV of 50,000. Thats a pretty big difference from what the guy wants and what it was appraised at.

I just want to make sure I buy right, since I believe this is when you make your money in the RE business. When you buy, not when you sell.

THANK YOU ALL IN ADVANCE FOR YOUR RESPONSE ;D

I know this varies from state to state, but I wouldn’t rely on assessed value as any indication of market value. If you have any appraisal, I’d go with what the appraiser came up with for market value–assuming the appraisal was done fairly recently, like within the last 6 months.