Sotheby’s offers to value council art collections

I don’t quite understand the problem with selling a few bits of the “family silver”. Many items in storage would never be missed. Reportedly the British Museum has so much it can’t even afford to document what it has & storage costs are increasing. Surely in times of funding crisis a few items could be sold from each Museum/collection in the country, and the money ploughed back into creative education, contemporary exhibitions etc etc. The country is skint – why not sell off a bit of old stuff from the loft which we never use/see anyway?