even out here on the Left Coast where many counties are online with taxes, there are disclaimers all over these websites that notify you that they may  not be up to date or accurate.

many counties only post secured property taxes and not their default or supplemental indices or other related secured taxes (redemption, escape, etc...)

it would be easy to miss a tax sale indeed because of this, not to mention any of the above taxes. 

example of a pitfall:  San Francisco County posts four different tax indices to review.  on a couple, the apn alone is insufficient as you need the tax bill number to access the data or the property address, so knowing what you're doing online certainly helps.

also, the county tax computers often go down for fixing or updating, especially during the payment seasons, so they are unreliable.

if the client wants to rely on third party or online County systems, it's their liability, but add a notation to your report that copies their email into it.

DO NOT DISCOUNT your services.  Continue to provide your base product regardless of what the client wants.  It's your due diligence.  If they don't want it, they can ignore it:  it's like a TV show that you don't like:  turn the channel.