If you just sell stamps then you will not get more than $1 or $2, but by making this kind of cover can get lot of money and can make your living on selling stamps and philatelic material.!!
How to make living on stamp collectors.!!
Also no one will notice whether stamps are forgery stamps (replica or facsimile).!! because of forged rubber stamp applied on this.
How can you tell this is genuine first fligh cover from peiping to Shanghai.?