And, for the 'art investor', as long as other collectors don't sell for a lower price, it doesn't even matter. Many galleries will blacklist collectors who sell for lower prices. As long as there's someone to buy, you're fine.
Most art sales don't happen through public auction. If you want to invest in more art, you'll have to go through galleries, which may choose whether or not to sell to you. And word does get around about who is bringing down the price of an artist.
And couldn't the person just hire someone to buy for them?