Welcome to the shadowy world that is Darth Nihilus.
You mentioned the CoTF Nihilus costume that regularly rests on eBay. Something to note about it is that if you want voluminous and gathered skirts you won't get enough fabric with that costume. I bought that costume and while I'm happy with the inner tunic, the outer tunic, and the cloak my wife and I are redoing the skirts and the obi. For me, the waist measurement of the skirts needs to be tripled in order to create the proper gathers for the CoTF Nihilus. The skirts that come with the costume come pretty close to your actual waist size and you can't get gathers out of that.
On the eBay costume, if you go that route, I suggest doing all of the weathering yourself. That way you have complete control over how the weathering is done. My outer tunic had the weathering started and there was a bit more to it than I would have preferred. Fortunately, I made the weathering work and improved it with a pair of scissors, a cheese grater, sandpaper, and a dish scrubber. The downside, though, is that my skirts and cloak will need a similar degree of weathering in order to look consistent. It's not bad, I just like having complete control over it.
The eBay costume will likely be made of black gabardine. It's a suiting fabric made of a wool and polyester blend. You'll be wearing layers of it so expect things to heat up. I can't completely comment on the amount of heat the costume generates as it still felt like summer here when I initially tried the costume on. It might by lovely in the winter but I don't know yet. The black gabardine, being partially synthetic, will respond to an iron. The polyester will melt slightly under the heat of an iron and you'll get a very smooth fabric. I suggest not ironing the costume. The weight of the gabardine keeps the costume pretty wrinkle-free.
The eBay costume doesn't come with any Velcro closures so you'll have to add them to the obi, the outer tunic (if you want Velcro to hold it closed), and the inner skirt. The skirts of this costume are separate pieces, unlike the costume standard's suggestion that the outer skirt can be attached to the inner tunic. The inner skirt is intended to be closed with Velcro and the outer skirt is meant to tie on like an apron.
The seller of the eBay costume is a nice young woman and she was always willing to chat with me and explain the details of the costume. So I have no complaints about her. The details I've written above come from my observations and experiences with this costume. I am replacing the skirts, which is pretty easy to do, though the fabric will cost me $104.00 (black gabardine is $13.00 a meter here and I need eight meters of the stuff). I want the look of a folded inner tunic and so I've bought a dickey made in black gabardine from someone else to wear between the inner and outer tunics. The eBay costume inner tunic wears like a long-sleeve pullover shirt and doesn't have the look of an Asian tunic (with one side folding closed over the other).
Hopefully this longer-than-expected post provides you with ample information if you decide to go the eBay route. Welcome to the boards and good luck with your costume.