I might go against most of the grain here but I recommend purchasing at a local retail store. Why? You have someone to help you figure out technical issues and potential upgrades down the road. Most airsoft products don't come with a warranty but in the few shops I have purchased they have helped me through a few bumps I have experienced. My experience buying second hand as a complete noob didn't go so well hence why I recommend the above.
Unfortunately it's against forum rules to post shops but a simple Google search will reveal a few.