I used to work for Oxfam, so I'm well aware of the storage problems charities have (the back room got seriously dangerous with sacks piled higher than my head). Part of the solution is to be ruthless about what you sell and junking the stuff that is not of good quality. The other part of the answer is staff training. Oxfam do book training courses among other things. I still remember waveney salvaging a first edition of John Masefield's 'Box of Delights' from a pile that another volunteer had earmarked for the bin. It later sold in the shop for a couple of hundred pounds. We'd already been on the book course and knew how to check for first editions. We also knew how to use the internet to see if those first editions were rare/valauble.

The other trick, which isn't always possible, is to recruit volunteers with the right skills. There's often someone around with an interest in pottery who can spot stuff like Clarice Cliff teapots - shops often don't think to advertise for retired people with specialist interests. They're often only to glad to have an outlet for their skills/knowledge.

I could tell the stand that had a pottery expert. Their carnival ware was sensibly priced (not horrendously expensive, but not giving it away either).