Binance (and other exchanges) have buyers and sellers. If you buy 0.0001 BTC, you are buying from a seller. If you are buying from the exchange, then the exchange is acting as the middle man.
When you first time bought the Btc from Binance, Binance took commision from transaction.
or How do you know the seller is Binance robot or real seller ?
questions?
In fact many exchanges had their own bots doing the trading. The market is mostly unregulated so there's nobody watching. If they want to manipulate the price or have their own "willy bot" trading using fake fiat balance they can pull it off. It's a bit risky because when Chinese exchanges were faking volume in 2014 the government shut them down.
Even if they're trading for themselves, what can you do? You can stop using them and use the nearest ATM that will later sell your coins at one of those big exchanges.