With cases involving artefacts with dubious provenance, every case is different & has to be handled differently. In this instance though it seems as though there may well not be a case at all. People do have to acept that like it or not, some artefacts have been acquired legitimately (or in this case, with no evidence to suggest otherwise). In many ways, people demanding that every artefact is returned to its country of origin merely serves to weaken the stronger more valid cases, along with spreading panic amongst museum directors.
Notwithstanding the above though – if auction houses always acted entirely reputably, then perhaps people wouldn’t be so quick to assume that they weren’t.
Daily Telegraph 
Chinese anger at sale of Qing Dynasty seal
China has reacted angrily to the sale of an 18th century Qing Dynasty seal by Sotheby’s in London.
By Peter Foster in Beijing
Published: 9:31PM GMT 06 Nov 2009
The green jade seal, belonging to the emperor Qian Long (1736-1795) fetched £3.6 million, six times its estimate, at the auction on Wednesday following frantic bidding by eight competing collectors.
News of the sale was greeted with anger on the Chinese internet, where the country’s growing nationalism frequently finds its voice.
“Bandits have seized our treasures and are now selling them off at auction for ridiculous profits. How can we tolerate such behaviour?” wrote one user of the Sohu Internet portal. “The Chinese government must get fully involved in this matter.” The seal, lot 136, was the prize object in a 261-lot sale which raised a total of £8.3m.
Although Sotheby’s said it was ‘not aware of any issue’ with the seal’s provenance – it was acquired in Paris in the 1970s by a European collector according to the catalogue notes – the auction revived memories of a controversial sale in March this year.
In that case, the sale in Paris of two bronze heads looted during the sacking of the Summer Palace in 1860, caused indignation across China, leading to diplomatic interventions by the Chinese government to try and halt the sale.
Last month China’s announced that it was mounting a global expedition to attempt to document lost treasures from the Summer Palace, including those held in the British Museum and Victoria & Albert Museum.
Sections of China’s state media urged caution when responding to the sale, pointing out that many lost relics were not looted, but ‘legitimately’ sold out of China for profit by Qing Dynasty officials.
However following Wednesday’s sale the State Administration of Cultural Heritages responded to popular pressure by again voicing opposition to the auction of looted cultural relics, and urging auction houses to comply with the spirit of relevant international treaties and professional ethics.
It also promised to expand support for Chinese organisation charged with studying, collecting and cataloguing China’s lost cultural heritage which was widely disseminated around the world during the colonial era.
A study by UNESCO, the United Nations cultural arm, estimated that there were 1.67m Chinese relics in 200 museums around the world, and up to ten times that number in private collections.