Having someone purchase me a handgun legal or illegal?

Well unfortunately for you the federal laws regarding licensed firearm dealers and your state laws regarding ownership of a pistol conflict each other. On one hand you can legally purchase a handgun, but it is illegal for someone to "straw purchase" it for you and an FFL at the gun store won't sell you one until you are 21. You might as well wait and just buy something nice at the gun store yourself rather than be limited to what is in the clified ads. I'm surprised you didn't find any good deals there though, that is usually where the deals are.