The Buyer's Quandary

Authorities are trying to recover a hoard of antique coins that were uncovered without permission near a river in Jiangxi (Police ignored as Chinese treasure hunters dig up 500kg of Qing dynasty coins  Thursday, 05 January, 2017):

Hundreds of Chinese treasure hunters – including pensioners and children – ignored outnumbered police as they dug up more than 500kg of ancient coins during an unauthorised mass dig near a river, mainland media reported. Local police had to bring in 20 reinforcements the next day before they were able to cordon off the area and stop villagers digging for artefacts in a 30 square metre area of riverbank beside the Gan River in Xingan county, Jiangxi province, last week, the provincial news portal Jxcn.cn reported. The county authorities are now trying to recover the antique coins, believed to date from the [...]  reign of Emperor Qianlong (17[35]-179[5]) during the Qing dynasty[...] The head of county’s cultural heritage bureau said local authorities were working with villagers to recover the coins, which were thought to have been part of a Qing courtier’s fortune, which was lost when a ship sank as he was travelling home to Jiangxi after his retirement. The authorities plan to carry out a further archaeological dig at the site, the report said. 

The private excavation of antiques is illegal as Chinese law stipulates that all such discoveries belong to the government. And when they 'surface' on the market without any paperwork, the looted coins will look like this....

Chien Lung, era chien lung (1736-1795). Obv: chien lung tung pao

 
... so, much like the unlooted coins which have no paperwork because previous sellers 'lost' it. So how can the collector avoid buying looted coins without checking the paperwork? Anyone care to answer? Thought not.