I'm doing both buying and selling in an app that I wrote. My app monitors open positions, closing them when certain conditions are met. Sometimes it buys replacements for closed positions. It also monitors total p&l and bails when the loss reaches a threshold. All orders, buys and sells, are market orders for NYSE stocks. The app manages two accounts simultaneously.
The app was written in Delphi/Pascal and accesses the TWS via sockets. It runs on a clean, fast computer that is used only for trading.
[rwk]