Coin Collectors?

Hi I'm wondering if anyone here is a coin collector, if so where's a reputable place to buy and sell coins, even auction houses would apply. I have a friend that's looking to sell some rare million dollar coins but doesn't know where to go, I won't say more than that unless it's in P.M.'s, everything is completely above board, but I don't want people outside of the community reading about it... Any info would be appreciated, please answer in P.M.'s.