This is about their state tax and the state constitution. It doesn't have anything to do with the federal capital gains tax or the federal constitution.
The complaint was that the capital gains tax was not uniform because it's an extra property tax. But this decision is that it's an excise tax on the sale, not the underlying value, so it's legal.
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[ 1.6 ms ] story [ 12.5 ms ] threadand the ruling: https://www.courts.wa.gov/opinions/pdf/1007698.pdf
The complaint was that the capital gains tax was not uniform because it's an extra property tax. But this decision is that it's an excise tax on the sale, not the underlying value, so it's legal.