lol i think you can trust all the same. no exchange wil try to scam you since that would be just bad advertisement. No lock that cant be broken exist on Earth, so all exchanges can also be hacked. But if you wil think only of this you will not get nowhere. Just pick exchange, and buy. put on your wallet and forget about it for a year or 2.

BTW, i never saw on Poloniex anywhere close to 500 BTC offered to buy XMR as are now. 200 BTC was for over 0.0039.