As Brian said, yes, Idaho is a non-disclosure state. There has been a lot of discussion lately about this very topic. Some people think we should go full disclosure, others think partial, others think we should stay non-disclosure.
Full disclosure (or even partial disclosure) can make it some what easier for someone that is not a member of the local MLS board to establish a value on the property. Easier, but not necessarily more accurate. Here's why:
1. First, a market value on a home is heavily influenced by recent sales. But that's not the only thing.
2. It's also influenced by tracking homes that were available for sale but didn't sell.
3. By tracking homes that are currently on the market and their 'on market' history.
4. By tracking prices (whether they are going up or down) within a comparable home type/size/area and within comparable home price ranges.
Keeping the sales price of a home private does not hinder appraisers or home sellers in establishing value. What it does hinder is the local tax assessor. In the past, the assessors office had a paid position that went around to properties and did 'assessment comparisons' establishing assessment values for taxes. The wonderful thing for non-disclosure, the assessors office couldn't use recent sales to 'reassess' every other home on the same street. This allows for lower assessments the longer someone has lived in the home. Sort of like the CA 'grandfather clause' they have on their taxes, without creating a 'law' to make it so.
Here's the problem with changing from non-disclosure to any other type of disclosure - transfer taxes. In most other states, transfer taxes are charged on real estate sales. Sort of like a 'sales tax'. There are varying degrees of taxation. We've personally sold property in other states and we've had to pay anywhere from 1-3% of the sales price of our home as a 'transfer tax'.
Currently, the average home seller pays approximately 7.5% of the sales price of the home in closing costs including remaining taxes, escrow fees, title insurance, real estate commissions, pre-payment penalties, etc. Making buyers and sellers disclose the purchase price of a home serves only one purpose, to eventually implement a transfer tax on the buying and selling of real estate.
You are correct, Idaho is a non-disclosure state, however unless requested by the sellers agents will enter the sales price of homes into the MLS system. That allows agents to be more accurate in valuation of real estate.