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https://buyairlinemiles.com/ is it legit or ok to do this?
If you're referring to the "process" of buying miles or points...Yes, that is legit and okay to do. People do it all the time.
HOWEVER....I have never heard of anyone using a page like this...and with the bold "WhatApp" phone number at the time does make me think it's kinda "scammy" of a site.
If you really want to buy points with one of the travel rewards programs you're a member of, I would check direclty with the hotel or airline to buy from them directly. That would be WAY MORE legit than using this site/service.
Buying or selling points or miles through a 3rd party broker typically violates the progam's T&C and getting caught puts you at risk of forfeiture and being booted out of the program but but it is legal. I can't speak to whether or not this particular broker is legit but I'm also more used to seeing brokers also advertise that they're looking to buy miles.