The conductor's not pocketing anything! You pay for the upgrade and get a bupiao, printed out on a little machine they carry. Not at all uncommon to hear announcements on a train that extra sleepers are available for upgrade from the conductor.

Please point us to that website you cite. Conductors now cannot sell a no-show berth, since the ticketed passenger might be boarding down the line, but they are free to sell unsold berths.