MacOS High Sierra : Xcode failed to download Use the purchase page Error during installing Xcode 9.3
I had the same issue and finally got this working, however I'm not sure exactly which step made it work, so I'm posting all of them.

From How to fix / Reset App Store app on Mac OSX - El Capitan:

Delete:

~/Library/Caches/com.apple.appstore
~/Library/Caches/storeaccount
~/Library/Caches/storeassets
~/Library/Caches/storedownload
~/Library/Caches/storeinappd


I didn't have some of those folders, but I did have a couple of extras that looked fishy, so I deleted them too:

~/Library/Caches/storeassetd
~/Library/Caches/storedownloadd


Run sudo softwareupdate --clear-catalog [Note: I did this but I don't think it helped]

From http://osxdaily.com/2016/10/08/mac-app-store-temp-cache-folder/ (paraphrasing):


Quit out of the Mac App Store
Open Terminal and run open $TMPDIR../C/com.apple.appstore/
Hit Return and the com.apple.appstore folder will open in the Finder of Mac OS
Delete the contents of this folder


And finally:


Navigate to ~/Applications in Finder and move Xcode to the Trash [Note: This step was necessary for me, but ymmv]
Reboot
Launch App Store. Search for Xcode. Install it fresh.