Maybe you've heard or read about it that exchanges do block those deposits from mixers or casinos. With those, they're not really dirty coins.

And even the freshly minted Bitcoins, won't be considered clean. I remember this idea came out for the first time I read it several years ago.

Where do you live? I don't think that someone is going to sell BTC under the market's price unless that person is in a hurry and needs money badly.