Frederic Thut told the British newspaper The Guardian he would not have agreed to the Saturday auction of the piece, titled "Slave Labour," if there had been any indication it had been stolen from its London location.
"It has been said that the artwork was stolen, and that is just not true," Thut said. "We take a lot of care with our consignors, who they are, what they do, and if there's any illegality we will not touch it. Everything is checked out 150 percent."
"Slave Labour" suddenly disappeared from the wall in north London only last week. It had appeared just as mysteriously in time for last year's Royal Jubilee.
The covert removal of the 4-by-5 slab stunned neighbors and British art lovers, some of whom gave Thut a piece of their mind in a steady stream of phone calls.
Thut said the person selling the mural was a well-known art collector who wanted to preserve the work, which was expected to fetch a six-figure price.
"It's about conservation," Thut said. "It could have been destroyed."