So there are 2 things there. There's payment for order flow - Robinhood will route your order to Citadel and Citadel will choose whether to trade with you at th

So there are 2 things there. There's payment for order flow - Robinhood will route your order to Citadel and Citadel will choose whether to trade with you at that price (or route your order somewhere else), the big difference though, is that Citadel is legally obliged to give you the NBBO (national best bid offer) so whilst Citadel is paying to be the counter party on your trade, it doesn't change the deal you get. It's no worse for you - you still get the best price, but there are various reasons Citadel wants to take the counter-side of that trade. HFTs are again, not able to effect your trade. If you place an order in an exchange then HFT's will respond once you have traded, but your trade is guaranteed to go into the market and execute without anyone interfering before hand. If there was liquidity in the market when you tried to trade at the price you entered, you will get that trade. Now it may be that an HFT sees your trade in Chicago and goes off and trades on some other exchange super quickly based on the new information or they might cancel their remaining orders in the makret, but they cannot interfere with your trade before execution. What's unique about the ethereum situation is that different actors int he market can interfere with your trade before it executes because your desire to trade gets published before it's executed and the miners (in this case I guess sort of analogous to the exchange) are willing to prevent you accessing the market due to a bribe from a third party based on the information you were forced to publish. Which means you never have confidence that you can actually execute a transaction you beleive is available in the market.