I'm just going to try to clear some things up for you.
Like everyone's previously said, you gotta get AV'd first. Once that happens, you'll have access to the Classified section, which is where all the buy/sell of old guns are done. As for "Black" guns, I assume you just mean all metal. These guns, like I said, can be found in the Classified section.
Just to note, the reason why you aren't finding alot of "black" guns on the market, is because it is hard to import them. Some retailers have begun importing guns with clear plastic lower receivers, which I'm assuming is what you're talking about, and they do this because these guns can pass through customs with relative ease. As people have mentioned, yes, you can get a hold of metal lower receivers to replace the plastic ones, and generally, some retailers will be able to get them to you.
As a suggestion, you should look up tokyoseven's "Noob FAQ" or something similar, and have a look at those links. They have all the valuable information you need concerning airsoft and the law.
I knew a man once who said "Death smiles at us all. All a man can do is smile back."