I always clearly communicate to the client they are responsible for estimating the fair market value of their non cash charitable contributions. If pressed, I ask them to estimate the value they could receive if they had sold the good at a thrift market. I also remind them they should be comfortable defending this value in the event of an IRS audit.

You are only asking for trouble if you take it on yourself to estimate this value on behalf of the taxpayer. This is their job, not yours.