Since there is such as thing as metric tubing, fittings, bolts, etc used in most of the world and you're buying directly from China, it makes sense that you'll find a lot of metric stuff out there. The easy answer is to stop buying metric fittings. Save a few bucks and you've traded that savings for hassle. There really is no cross reference between mm and inches or at least not at the tolerances most fittings need to work together. As I said a few times though, you CAN replace the compression ferrule with a stack of orings to lock down on a slightly undersized tube in a pinch.
I'll get up on a soap box a little. Plumbing parts, NPT, Flare, Compression, QDs, all come in different sizes and the inter-working between parts is not necessarily an amateur's game. I've been selling these parts for 8 years now and when you buy my parts at the slightly higher prices than Hong Kong Ebay, you're getting my labor of vetting component quality, correct sizing, and then pre and post sale advice. I know it's extremely tempting to bargain hunt. Ok, that's all.
