A buyer can reserve a maximum of two crates of 24 bottles for between $60 to $75 a crate. They must pick them up at the abbey gate at an agreed date and time after handing over their car number plate.
However, Jan Linders risked divine vengeance by selling the beer for $17 a bottle, almost five times that charged by the monks.
"A price of almost 10 euro per bottle is contrary to the ethical standards and values where the monks stand for," a spokesman for the abbey intoned.
"Every beer lover knows that the Trappist monks of Westvleteren do not pursue profit maximisation, they produce just as much beer as necessary to make a living. All profits made by the abbey are donated to charity."
A Jan Linders spokesman told de Volkskrant, a Dutch daily newspaper, that the beer had been obtained to reward loyal customers.
The paper reported that the 300 crates of sought-after beer had flown off the shelves.
The spokesman claimed the chain had made minimal profits because they had to pay off their suppliers, but refused to identify the shadowy middlemen who may have hoodwinked the holy men.
Gineke Wilms, the company's marketing manager, said: "The beer was purchased through a number of links, which is why the price was this amount. We emphasised to the abbey that we had really good intentions. We respect the exclusivity of beer enormously."
An abbey spokesman said: "We especially hope that it will not happen again."
Saint Sixtus is one of the 11 Trappist breweries in the world and one of the six in Belgium. To qualify as Trappist, a beer must be brewed by monks or in the walls of a monastery and a proportion of profits donated to good works.