Henry George didn't say that only land had value, he believed that society was the cause for the increase in value of unimproved land, and that this increase in value should be returned to society in the form of taxes.

And while it may be difficult to determine a reliable land value for taxation purposes, practically every community of reasonable size already does so, in the form of property taxes -- which are simply taxes on the bare land plus taxes on improvements -- and when they use eminent domain to take over a property for public use and pay fair market value. If anything, figuring out the value of bare land by itself would be easier, since you wouldn't have to consider improvements at all.

And for land development, it is possible to separate private improvements from publicly-financed improvements -- or even, when private improvements raise the value of surrounding property, to give a tax rebate based on the increase in collection from one's neighbors.

Finally, even if setting a fair value for unimproved land is difficult, the tax itself is far easier to collect and enforce than sales tax, particularly a sales tax high enough to encourage cheating or lobbying.