it's an interesting idea, but seems this ATM would end up buying high and selling low.
someone loads the ATM with 10BTC the ATM gives him 6,000$ cash, then that ATM is considered overload in BTC so it offers to sell the same 10BTC for 5,500$

you'd have to charge one hell of a fee to make buying high selling low profitable.
There's no such thing as "overloaded in BTC" - bitcoins don't take up space. Giving up cash would either cause the machine to increase the price more slowly, or reverse and start lowering.
You'd need to maintain a spread between the buy and sell prices, and probably also limit the amount of a sale such that you'd never get the situation where the customer could arbitrage the machine itself.