You're worried the $101 isn't actually there - i.e. if you place an order to buy for $102, either the $101 order will get pulled by the HFT trader, or the HFT t

You're worried the $101 isn't actually there - i.e. if you place an order to buy for $102, either the $101 order will get pulled by the HFT trader, or the HFT trader will buy the $101 order and sell it to you at $102. If there is a sell order @ 101 and you place a buy @ 102, the exchange will instantaneously match the orders. The exchange will then inform the HFT and you (simultaneously) that a trade occurred. I repeatedly linked to an HFT tutorial which explains this. Please read it.