My day job is in the archives and museums sector so it might be useful if I gave an insight and explanation from the 'other side of the fence'. I have no knowledge at all of Roger L's collection so these observations not specific to his collection in the hope they might be useful to aditnow members more generally.
These notes pertain to public museums and archives. Privately owned collections are different entities that might potentially be sold or dispersed when current owners retire, die, or experience financial issues. Broadly public collections will be in the ownership of local authorities (e.g. county councils), national bodies (e.g. the various national museums) and, increasingly, charitable trusts as some local authorities due to austerity are hiving-off non-profitable activities to trusts. The long term survival of trusts is an unknown matter.
Public museums and archives have collecting policies which set out what they will collect and what they will not. These are public documents and will be online in some cases and available if requested if not online. Collecting policies will define the geographical area the institution will collect from - e.g. county museums and county archives will collect material from or relevant to the county that they serve. Collecting policies for specialist institutions such as the three national coal mining museums for England, Scotland and Wales will also have subject limitations - in this example it will be the coal industry, coalfield communities, etc. but not, I presume, non-ferrous metal or slate mining. A few institutions may have time limits to what they collect - e.g. a Roman museum.
Long-established museums and archives tend to have large collections with commonplace items already well-represented. Most institutions have far more in reserve storage than on display due to either limited size of display space, or reflecting that some collections are not prime display material - e.g. a lot of archaeological finds are useful for research and study but only the best preserved are generally interesting enough to most visitors to merit displaying. The considerable costs of collecting, documenting, conserving, indefinite storage in climate controlled stores, and display, limit what can be added to collections. Very few institutions have sufficient resources to collect on a large scale and without being selective. This has always been the case, and public funding cutbacks in the ongoing period of austerity has exacerbated this.
Objects, documents and photos collected by museums and archives must all be conserved and stored to the same high standard, except for limited specified 'handling' or 'education' collections which are separate to the main or 'permanent' collections, and will in a sense be 'sacrificial' as objects handled by school parties and taken to events are at heightened risk of damage or loss. Institutions accepting donated objects should always make clear to donors or sellers which collection the material will be going into.
The policies and procedures that public museums and archives work within are purposely framed to make it difficult to dispose of material. Disposals from the 'permanent' collections have to have approvals from senior management and boards of trustees, with detailed justifications as to why disposal is proposed. Typical reasons are excessive deterioration or infestation or a review of the collection having shown that a portion of a collection is well outside the institution's collecting field. Public museums are obliged to consult with the original donors (if they can be traced) and to offer the material to other museums for free through a listing in Museums Journal; if no expressions of interest are received then non-accredited museums and collections can be approached and as the last resort the object being disposed of can be offered for sale or, if there is little prospect of sale, it can be dumped. This laborious procedure dissuades most institutions from disposals unless there is a very good reason to prompt all this work.
Many public museums and archives will decline xeroxes and other copies of maps, photos and documents that are copied from material in other public collections. This saves them from the cost of preserving copies for ever when the originals are available for consultation in other public collections.
Larger archives and museums generally already possess the 'obvious' material within their collecting field and will not generally seek to collect further multiple examples unless they have some especially important connection: e.g. a large mining museum will probably possess multiple examples of the standard types of twentieth century miners' lamps and would probably only seek another duplicate if it came from a major event such as a mine disaster or had been used by a particularly well-known person.
Private collectors may seek a home for their collections in their entirety. However private collections often are much wider in scope than the collecting fields of the museums they approach, and may contain a good deal of material that is already well-represented in the public museum's collection. It would be unusual for the public museum to agree to take the entirety of the public collection in such circumstances. Instead, the museum would probably seek to be selective in what it sought to collect, concentrating on filling gaps in its existing collection, and not seeking to collect material that fell outside its collecting field - which might be geographical, time -based or subject-based, or a combination of such parameters.
These notes and the hypothetical examples they contain are general, to help inform aditnow members. I have no knowledge of Roger L's collection and none of these notes are observations on his collection or on the museum he approached. I hope these notes may have been useful.