The old docking blocks are supposed to be unbuyable at shops, only sellable.
This is the case when using the standard buy button, however, when using buy-amount one can still buy them.
To reproduce:
# find a shop
# attempt to buy one of the old docking blocks using the 'buy' button
# observe as you cannot buy it, which is the intended behaviour
# use the 'buy amount' button to buy multiple old docking blocks
# observe as you obtain them, which is not intended