For this reason alone, Betfair has no reason to ban or close your account when you should win. This is what Betfair first became famous for; peer-to-peer betting. Whilst the Exchange account won't be closed for winning, there is an additional commission rate.
Trading the Betfair markets is most definitely a genuine earning opportunity and for those who are determined and committed enough, it certainly can and does replace a full time job. And then some.