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Although quite unlikely you could have similar weight and fit requirements than the seller. But of course buying someone else's custom is not going to bring the main benefits that the original buyer enjoyed.
Anyway going back to the subject of the relevance of custom bikes (in general), @swisschap told us that something designed especially for you can't possibly be better than something made for no one in particular, so that's us told.
In general (if you were to get a custom frame):
Of course if you buy a bike made for someone else the first 2 points might be moot.