I have one of those. Showcarsrob was in the business awhile back, but collected bunch of negative customer feedback. Not on eBay so much as through his regular business. I bought one through eBay from him recently though and had no issues, even with shipping from Canada.
As far as the quality, it's pretty good for a fiberglass part. It will take a little finish work if you really want it to look nice, but that goes for pretty much every fiberglass part I've ever seen.
I also have an original DC spoiler, and another fiberglass version that doesn't have the screw pockets for mounting like the DC and the one showcarsrob makes. The DC version is ABS, thin, and very flexible. From what I understand the originals are somewhat prone to cracking, and were only attached with plastic push rivets originally. The DC version is slightly wider than the version showcarsrob makes, probably by a 1/4", so there's a little more trunk lid visible around the edge of the spoiler with the showcarsrob piece. Doesn't look bad, just a little different. And honestly, it would be pretty easy to take care of when you're prepping it for paint anyway.
The other fiberglass version without the screw pockets is lower quality. It would take a LOT of work to get it to fit properly. It's also heavier than even the showcarsrob version.
Of the three, I actually think the showcarsrob version is the best for actual use. The DC version is a better fit, but I'd worry about it's durability if the car gets used a lot (like mine does as a daily driver). The showcarsrob version will take a little finish work, but it will last a long time after that's done.
Just my observations. One of these days I should line up all the ones I have, take some pictures, and make a comparison thread. The topic comes up enough...