Charities are taking on more volunteers with specialist skills in valuing items donated to shops and investing more in training existing volunteers to develop such skills, according to the Charity Retail Association.
Isabelle Adam, project and policy officer at the association, said more charities were becoming aware of the importance of having specialist knowledge in their shops.
She said an increasing number of charities were making contact with specialists, such as antique dealers and jewellery valuers, so they could ask for their help in valuing items at their shops on a voluntary basis.
"More and more charities are encouraging volunteers to put something aside if it could be of interest and giving them awareness training of what might be valuable," she said.
A spokesman for Oxfam said it had a number of specialist volunteers who chose to help the charity because they knew their skills could be of use. For example, he said at the Oxfam book shop in Oxford, a number of professors volunteered because they knew a lot about valuing books.
"We also set up a team called Valued a few years ago, made up of paid staff who are a team of specialists that shops can send items to for valuation if they are unsure what the item is worth," he said.
The Oxfam shop in Newbury recently advertised successfully for a volunteer with specialist knowledge of sheet music shortly after a rare score by Mozart was donated to the shop.
As was customary for all sheet music at the time, the piece was sent on to the Reading shop and it was valued at £3,000.
Janet Bates, manager at the Newbury shop, said that taking on a volunteer with specialist music knowledge was unrelated to the Mozart incident, but it was important to find someone with such expertise because musical items were often donated to the shop.
A spokeswoman for the British Heart Foundation said when shop managers went through their induction they were trained to keep an eye out for things that might valuable."They can then contact our team that sells items on eBay if there is something of value," she said.