Of course, there are imperfections in some assets. Let's not talk in the abstract. We've established that this problem applies to 5.8% of assets. Some percentage of those have reasonably computable valuations. If not or if the valuation is contested, we can allow tax to be paid on profits as an alternative or a cumulative tax to be paid at sale based on average value through the holding period which could be inferred given a standard growth curve.

Real-estate appraisals are imperfect but getting within 20% most of the time is quite achievable. The main problem is for unusual or expensive properties which could be handled similarly to hard-to-value assets.

Your statement doesn't follow unless defining that subset is easy. What should be included or excluded? As we're discussing corporations, we have ample evidence of the ability to game. You could suggest closing loopholes but most of the loopholes also have legitimate uses.

You write that "capital assets are valued at sale for income purposes" is a solution to the difficulty in measuring their value. Why is this the case? Remember that the origin of this discussion is whether it's better to tax corporate income or the wealth of those who own the vast majority of that income. Are you asserting that valuing a person's equity holdings is difficult?