Yes, there is a huge difference in quality. It relates to manufacturing tolerances, mechanism quality, plating, quality control..... the list goes on.
I've just used the example of a sierra in another thread, and it's a handy example here too. Take a sierra from more or less any ebay seller or any online seller which has a name akin to penkits dot whatever. It will be Chinese for sure. Take a Berea sierra, who are the people that designed it in the first place, (which you can buy from Turners Retreat), which is Taiwanese. First compare the centre bands - the former looks like its been embossed with a blunt stick on a piece of tin foil - the latter is properly engraved. You can spot the difference a mile off, even in just a photograph.
Next, look at the mechanism. You can't spot that in a photo, but side by side the difference in quality, both in appearance and in operation is enormous. The former looks, shall we say, shabby, next to the latter - looks aren't everything when it comes to a mechanism, but that translates to the operation too. The former is looser and wishywashy, so is its screw thread connection to the nib section - the latter is crisper and tighter.
Look at the way the two press together. Look at the quality of the plating. The difference between them will be obvious with the kits in your hand.
In terms of variety, kit designs are driven by a small handful of designers/manufacturers, who include Beaufort Ink (of course) Berea, PennState, Dayacom, and to a certain extent, Timberbits, all of whom have manufacturing arrangements with factories in Taiwan, because the quality of manufacturer there is much, much higher than in China. There are one or two other manufacturers who make in Taiwan too, but their kits are mainly just copies, (and just to cover my own back, there may be one or two people on the periphery as well). The Chinese then come along and copy, then sell them through anybody who is prepared to buy and re-sell, which they often do from their spare bedroom. The main Chinese supplier is called Rizheng, and anybody at all can set up a pen kit selling business re-selling their products, which as I say, are more or less all knock offs of kits designed by those in the list above.