When you exchange Bitcoins for dollars at an ATM, it's the same as when you sell them any other way: the coins become the property of the ATM owner, who can do with them what they like. They might:

• Sell them on an exchange (to know which one, you'd have to ask them, but there's really no reason they should tell you).

• Keep them to sell to future ATM users who want to buy coins

• Hold them for speculation

• Use them to pay employees, or buy other goods and services

• Draw pentagrams on the floor, light candles and chant the digits of the private key

There's no way for you to know what they do with the coins unless they tell you, and no reason why you should really care.