We all felt daunted by the prospect of building armour. There are plenty of people here in FISD that can guide you every step of the way. One line I here repeated often in these forums is, "most mistakes are fixable", so, in saying that, you would be better off, doing your research and buying armour from a vetted maker here on FISD. It seems the ebay guy is not forth coming with answers to your questions. This rings alarm bells to me.
If you are going to do it, do it the right way and avoid sellers on evil bay .