Computerized / Automated Valuation 

These systems can provide a wide range ballpark and are often free. These have two great purposes.

When to use an AVM. 

One - an idea of home value within about 15% when a general idea is needed, but these decisions will not have a financial impact or to keep up with an idea of the market over time. 

Two - Refinancing. These are great for refinance situations where ownership is not actually changing. Generally, the banks look for  80% 20% value on the loan, which reduces the lender from going in too deep with the lending. 

These do not have a risk of financial harm to a seller. Zillow is a great example of AVM simply estimating based on public records and often a house in proximity to the subject house - rather than accurate records and the most compatible comparisons.  

When not to use an AVM.

However, for a listing, this does not offer an accurate evaluation for obvious reasons - they cannot provide major information. For a buyer, this not verify the buyer is making a reasonable offer. Both parties are at risk. The property may not appraise. 

This is the least accurate for various reasons, sometimes off by tens of thousands, but can still give a wide ballpark figure. This uses combinations of often unchecked public records and homes that are in the proximately to the home being evaluated rather than the most comparable. These leaves room for a great deal of error when it is time for the offers to come in by an experienced agent and/or the appraisal that seeks accurate comparations.