If they are the exact same metal and design then yes, brand doesn't matter. But what you're paying extra for with a name brand item is knowing that the type of metal and design of the part are proven. An ebay listing can say whatever type of metal they want, but most people have no way to ever check. For example there's a huge difference between plain 6061 aluminum and 6061-T6. And unless you carefully measure ebay parts vs name brand you don't know that the design is the same even if they look the same at first glance.
very true, that is why you always buy one that has a great return policy. Buy and check them out, if they are not the same metal, then return it, simple as that. I guess you are right and can say most people do not have the resources to check what kind of metal it is. But that's what I did, and my metal is in fact 6061-T6 aluminum, and I only paid $125 shipped for them. I bought them from a company in USA, so returns would not be a hassle compared to buying them from China or overseas. If buying "aluminum" from China, then I would be skeptical about them.