A neat bot would be one which automatically buys and then resells things with a markup. It would figure out the usual price an item goes for on a website like Ebay, and purchase it if the price is set far enough below.
I have always wanted to try something like this with stocks or commodities. Make a bot watch the market, and buy and sell. I even got far enough to imagine some sort of genetic filtering, aka give the master bot 10k, then it distributes it to 10 bots, all which are slightly different, they spend it, the master takes any profit above 1k and invests it in another bot that is a combination of the best 2. Etc. That way your max loss would be 10k, and you might make some money.