[djohn]

Investing in anything takes knowledge and luck. However running a business (i.e. a card shop) is not so much about investing as it is about the understanding the operations of the business, and being able to change with the times when needed (both in terms of who are the hottest players and getting cards of them in stock, and also in terms of technology; as most card shops now have some sort of online presence, be it a website or selling through eBay, COMC, SportsBuy, etc).
Now back to your original question regarding how to invest in cards. It is not easy and definitely requires quite a bit of knowledge and luck. Just buying old cards and having them graded may work, but it is not a guarantee of profit (as even old players' cards can drop in value). Buying new cards of potential future HOFers can be dangerous as well (look at many of the "sure 1st ballot" candidates from the 80s and 90s who now due to the PED scandals might not ever get in and whose card values have all dropped). Even current rookies have high risk, as the current style of investing seems to be to buy high and sell higher (in hopes that there is another person willing to pay a little bit more than you). Even established players like Albert Pujols can see some big drop in value if they have a bad year or as they get closer to the end of their careers and their yearly numbers drop. You can try the method of flipping cards by buying lots of no-name prospects and hoping one of them hits big, then selling when they do. You could try box/case breaks but that is more gambling then investing.
Now I don't want to discourage you from investing in cards, I just wanted to point out that you need to do some research to understand exactly what method of investing may work for you. Keep in mind most people that "invest" in cards do it because they enjoy the cards to begin with and see the possible profit as a nice reward but not the main reason for collecting.