Buying a used gun can be a great way to not only save money, but to not loose any money if reselling becomes needed. The fact is that guns hold their value quite well. They last for many decades, but their are problems with purchasing used guns.
For one, it is dependent on the gun's care as to how long they will last. You may take great care of your guns, but who knows if the person you purchased it from did the same? Because of this, you need to have some knowledge of how a good, working gun functions. You also need to be able to try out a gun via test fire and inspection before purchasing it. Because of this, I don't advise online used gun purchase, unless it's something like Craigslist where you can meet the seller face to face. Even this can be risky, as people have been known to be robbed. One great place who you can trust is your local gun shops. They usually sell used guns that have been inspected and cared for.
Whichever you decide, please make sure that you know how to properly handle and use your new firearm.