An antiques dealer spotted the advertisement in June and alerted Poland's culture ministry, which then asked the FBI to pull the item. The US ambassador in Warsaw returned the cap to the ministry last week.
"A seller whose identity we don't know put up the cap for auction on eBay for a starting price of $1,750 (1,300 euros)," museum spokeswoman Agnieszka Kowalczyk-Nowak told AFP.
"It's a really valuable museum piece because it's an authentic cap. Plus, it's in good shape," Kowalczyk-Nowak said.
The cap features an inventory number that identifies it as belonging to the museum collection.
"We know nothing about whoever wore the cap. It was a man's cap. The women of the camp wore scarves," said Kowalczyk-Nowak.
The cap was on display from 1962 to 1995 along with some 6,000 other items of prisoner clothing, she said, adding that security had been light at the time.
The camp suffered another theft in 1989, when a Swedish artist stole the ashes of Holocaust victim from a crematorium there.
The artist, Carl Michael von Hausswolff, said he diluted the ashes in water and used them in a watercolour painting he exhibited in Sweden in December 2012.
Swedish police dropped an investigation due to lack of evidence -- since the offence was committed abroad -- while Polish prosecutors decided against pursuing charges because the statute of limitations had expired.
In December 2010, a Polish court jailed a Swedish neo-Nazi for masterminding the theft of the infamous "Arbeit macht frei" sign from the entrance to the former Auschwitz death camp site, located in southern Poland.
Nazi Germany set up the Majdanek camp near the city of Lublin in 1941 and ran it until 1944.
The museum estimates that 80,000 prisoners, including 60,000 Jews, died there -- around half those who passed through the camp.
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