I want to get a 3% discount on buying 500 BTC.
Hey OP, please be very careful in dealing with anyone here. If you'd really like to purchase BTC, please do so in your trusted exchange. If you received a DM with offers that is too good to be true, ignore that offer and move on.
I think OP was just joking and not really wants to buy 500 Bitcoins or maybe OP himself wants to scam Bitcoin sellers. If he is serious then he should have bought BTC on market as today Bitcoin price is cheap enough to buy. Well, it's hard to easily believe people as anyone can be a thief.