Stolen artefacts from Turkey found in Germany

one of the missing objects

Stolen artifacts from the Kocaeli Archaeology Museum (Turkey) have been found in Germany (Ömer Erbil, 'Stolen artifacts from Turkey found in Germany', Hurriyet  Wednesday,February 15 2017),
Several artefacts from Izmir were stolen from the Kocaeli Archaeology Museum (Turkey) in 2009 in 2012, one of the security guards was suspected of being involved in the crime and  the museum director of the time was also discharged.

Turkey later informed Interpol about the lost artifacts and in 2013, German police informed Turkish officials that the artifacts had been put up for sale by the Sixbid auction house in Germany. The artifacts had been put up for sale on the Internet, and the Turkish Foreign Ministry applied to the auction house to return the artifacts. The online sale was canceled but the auction company did not accept that the artifacts were stolen. In a written statement, the Culture and Tourism Ministry said talks about the artifacts were still ongoing and added that it would take further steps if there was no positive result.

 It is unclear how 'Sixbid' can claim the artefacts are of legal origins if they were excavated just a few years earlier and were included in the museum inventory subsequently. What documentation do they rely on and who was the consigner? What checks are carried out to ensure such items 'surfacing' on the market have verifiable licit origins?