Its not legit, since selling chips mean transferring them by losing on purpose, also known as "chip dumping", which are against the TOS of all poker platforms. Sites also dont want this to happen, because they sell chips themselfes. Actually this was "big business" back in the day on Zynga Poker Facebook platform, where people would form "teams" (collude) or use bots just to win these fake money chips and sell them to other players.
As for why somebody will buy fake money chips, thats a good question. Logically this makes no sense, and probably the best answer is, that even play money poker can be addictive. Once people have reached a certain blind level, they feel, they "belong" there, and since almost noone uses proper
bankroll management, they go bust again and again.
Then they dont want to start all over with the few free daily chips, that are given to them. Hence they buy a bunch of chips either from the site or from players selling them illegally, so they can get back to the level, where they feel, they "belong". Or they even beg for chips from other players, which also happened all the time on Zynga. Kind of a crazy circus really, which I certainly dont miss