It's true with any market: Buy low, sell high. eBay is a good place to pick up some bargains, but you can also get burned. Before you start buying there, you need to plan for what you are able to do if the deal goes south. I've gotten many great deals on eBay, and I've also had a couple that weren't worth the hassle. If you have a sharp eye, lots of patience, and a little bit of luck, you can find good deals on eBay.
So you can buy on ebay and sell at a profit at a coin show. I would suggest that the opposite could also be true - buy at a coin show and sell at a profit on ebay. Neither method is better than the other and neither method is guaranteed to work. You could lose as well as gain. You have to know grading, values, market demand and what you're doing.
I agree. There is nothing wrong with buying at a show and selling elsewhere. I do it all the time. In fact, some of my better buys have come from shows, from knowledgeable dealers. You just really have to know which market will bear which prices, and have a good eye to boot.