If it's a 1 time thing you're almost 100% safe . have the person you buy from stake the money to you but just trading is usually fine they go for the seller rather than the buyer most the time.
Personally, I'd be careful. I bought 40m today, had it in my money pouch, and it reandomly disappeared. I feel as if I done got caught by fagex and they took my munies. Just a word of warning!
As people say, stake the money and you should be fine. However, when staking it, you should consider putting something of equal or relatively equal value. If all the trader does is put up 200M and you don't put up anything; it is still highly suspicious.