Investor program: You input an item and you have a buy and sell price. The program scours user shops and buys at or below the buy price. Then, the program runs once a day/week, and plots a graph to see how your investment is doing. It will give a ping when the investment has reached a sale price. That'll probably only work for smaller scale investments, though.
Problems I see: Who leaves their computer running constantly for that long? Who wants to leave a program running for that long. I sure don't. Solution: UserShop / Wizard Buyer to buy the items initially. When doing other stuff, check once a week or so to see the average price. Make your own notepad document or heck, make a graph in excel. I cannot see this being a popular program sorry. I cannot see someone making it for mass... You MIGHT be able to get a private version made, but that'll cost you.