Recently bought many antminers and 29% of them were obviously used.
PCB's, fan, aluminum heatsink were full of dust. I had to air pressure clean the heatsink and big chunks of dirt came out.
How can a big name like BITMAIN do this (from my reading, i am far from being the only one).
Is it an ancient Chinese tradition to sell used stuff as new ?

No more "ancient Chinese" than new American tradition. See BFL for case in point.
You might want to stick to the company and lay off the nationality / race / culture angle. No one likes the racial overtones even from a fellow Quebecois d'accord?