You might check the museum association of the UK's website, which states
under its Code of Ethics
http://www.museumsassociation.org/asset_arena/text/cs/code_of_ethics.pdf
:

2.14 Refuse to identify, authenticate or value items in private
ownership for personal payment, gifts or favours.

Private ownership would thus include things being auctioned or otherwise
on the market.  I am familiar with several US museums that do not permit
their curatorial or other staff to comment to donors, collectors or
others on the financial value of objects, expected appreciation or
depreciation of such items or related issues.  In part, it's because of
ethical conflicts; in part, it's because of possible liability issues
for the museum.  Discussing aesthetics is fine; discussing valuations
and the like is not.

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