There are certainly good deals to be had online. However, I think it's important that the dive community support local dive shops whenever possible. I have purchased smaller items online (reels, flashlights, line cutters, etc.), but I make sure to purchase all of my major gear from my LDS. Plus, unless you plan on servicing your gear yourself, they are the ones you will be bringing your stuff to.
You can buy used gear if you like, but I would urge caution. Buying a used reg set from your LDS, for example, is probably fine. Buying from Ebay on the other hand might land you with a reg that was never serviced or improperly serviced that might have more issues than it's worth. It depends on the gear, of course. Fins? Sure. Regs? Be careful.
Lastly, as many here will also advise, think carefully before you pull the trigger on any large dive gear purchases. It's probably best to rent for a while and decide what you like and what your needs are. If you will be mostly vacation diving in warm water for a week or two per year, you are probably better off renting. If you dive more than that, or if you dive locally, then it's a good idea to invest in your own gear. But renting first will give you a sense of what you prefer. Back inflate BCD, jacket style, or BP/W? Do you need cold water regs or will you do fine with something like the Aqualung Calypso? Do you want air integration for your dive computer? Will you be using a transmitter and an SPG, or just one or the other? That determines not only the dive computer you choose, but how many HP ports you need on your reg first stage. What type and thickness of exposure suit do you need in order to feel comfortable in the water you plan on diving? These are just some of the questions you will need to consider and answer before taking the plunge into buying gear.
Good luck!