First gotta finish closing thingies in Orb vallis so I can get my shiny new bfg.I do hope that both the OP and anyone who purchases from him/her is aware that this sort of transaction is EXPLICITLY FORBIDDEN by Warframe's terms of service - the one and only place you may buy plat for real money, is the official Warframe website.
As such, if you log in and find that the plat you just bought from OP has been removed from your account, do not be surprised or upset - consider yourself lucky that you didn't get permabanned instead.
Hit up The Index.
Also as far as the trading/selling of plat goes, unless the plat was illegally acquired, DE has no reason to remove the plat from buyer's account.
Huge amounts of plat gets traded daily by people for rivens and the like.