Why some carbon fibre is so cheap?

Interestingly, the desperation to get a carbon frame is no more obvious elsewhere than on Ebay. People will settle for any junk just to get "carbon"! Ignorance is bliss, and sellers usually take advantage of it. Motobecane, tengusa etc are a few of the brands. For some of these, you don't find a website outside of Ebay, nor can they compete in the market.

I strongly believe some of these are not "real" carbon frame. They are just donor aluminum or fiber class wrapped with a roll of carbon fibre somewhere in Taiwan! Anyone can do it..

So beneath all the nice decals and artwork, you are essentially riding on junk. The frame will have properties inferior when compared with true carbon frames.

Good carbon frames will cost significantly higher, but that price is because of engineering time and labor, capital costs (such as shape giving carbon molds that cost thousands of dollars), amount of resin and density of weaving used. Another factor is the type of frame. Some can be a fusion of carbon and aluminum, some can have aluminum lugs in critical areas of attachment (lowering production costs), others, like most high end frames, are one piece monocoque like an airplane fuselage.

To put the capital costs into perspective, which I read on a website I don't remember now. To make carbon frames, you need molds for the sheets to be inserted into. If you're considering 4 or 5 frame sizes you need to multiply that amount by at least 4 in order to offer 1 carbon frame in 4 different sizes! So to start a carbon frame making business, you need some serious moolah!