Great question John. I've bought cards strictly for resale a couple times, but I keep coming back around to just trying to enjoy my cards without over thinking the profit side of it. Trying to buy and sell cards for profit suddenly starts to feel like too much work
That's another cool aspect about this hobby...you absolutely CAN use it to generate a little extra cash, and it appears that there are some people on this board that are making a living out of it. And that's awesome!
But I've realized I personally have more fun acquiring cards I like in slightly lesser condition to look at them in my albums, research the players, etc. I do try and be sure not to overspend on the cards I buy, because I'd love to be able to at least sell my cards for near the same price I bought them if I ever quit enjoying them.