I began researching the British X Corps crossing of the Garigliano River some time ago. Keeping an eye on E-Bay for anything interesting I spotted a full set of documents from the RAOC Officer in charge with detailed plans, overlay's etc. (including his Diary). It was eventually sold to a private collector for a huge sum. I was also talking to a man who does house clearances who said that very often he, and others like him often pick up this sort of thing. Some throw them away, others who realise what they are, sell them to collectors usually through e-bay.
Why cannot there be a register of these sorts of documents that are now in private hands, so historians like myself can look at when conducting research and enable us to access them.