Not sure I’d ever be able to pluck up the courage to go for the super cheap eBay specials but buying from one of the established distributors like HongFu, DengFu or GotoBike is a safe bet. (Look for CEN certification for peace of mind).
(Not a 29er but I got a TT frame and wheels from GotoBike: http://www.spanner.org.uk/2011/05/gallery-ws01-carbon-time-trial-frame-and-wheels-from-gotobike/)
But… given the hassle you might find that buying from On-One is easier and a very similar price (I think they’ve got some SS 29er’s in their clearance frames at the moment).
12K and 3K refers to the weave of the (cosmetic) top layer of carbon, 3K is the more familiar ‘carbon’ look with 12K being a much larger checker board effect.