Probably the best way to do this that would actually be fair and as accurate as plausible would be to auction some sort of contractual instrument that pays out

Probably the best way to do this that would actually be fair and as accurate as plausible would be to auction some sort of contractual instrument that pays out or charges the bearer depending on the difference between the sale price (whenever the land happens to sell again, if ever) and a predicted price (set by the auction). The land would be taxed at the rate established by the auction, and the instrument would be configured such that manipulating the auction on your own property is economically unfavorable. This would allow people with expertise in accurate land pricing to make (a bit of) money and remove any arbitrary or unfair pricing from a tax assessor.