I've been on the fence about rep carbon stuff from ebay, and from reading the feedback from the various vendors it seems like a lot of it is at least satisfactory for the price. If the price is cheap enough to justify I'll buy something and post about it. I don't want a diffuser though, really just a front lip/splitter and a rear performance style spoiler.
I think it is a fairly safe gamble though. Worst comes to worst you can leave negative feedback and likely get a refund.