In a previous comment, Paul Barford suggests that collectors should refuse to touch coins with no documented history as a way to fight looting.
Rather than signalling preferences for documentation perhaps collectors could explicitly pay dealers for details leading back to the source? If a few collectors stop bidding on undocumented coins and start bidding higher on well-documented ones many dealers wouldn't even notice. If I actually offer $10 or 1% on top of of the purchase price for provenance information they might pay more attention.
I'd pay a lot to know where the dealer got many of my coins from, but I'm mostly interested in the coins of uncertain mints. For common coins I'm less curious but I might still pay a little.
In some cases the provenance info might be a photocopy of a Harlan J Berk ticket from the 1980s. Maybe I pay just to learn that the coin came in a zip-lock bag from a gentleman with a Lebanese accent at a recent coin show.
It isn't clear what the social norms on tracing coins back should be. I purchased two coins that were published in SNG Levante from CNG. One had a further history published in Levante's SNG but the more interesting one did not. Mr. Levante is still alive. Would it be appropriate to write to him and ask him if he recalls where he bought a coin in the 1980s? Although you would not know it from my posts here I'm a bit shy about this sort of thing.
Collectors are at the end of a long chain from the soil. It's hard to signal ethical values up the chain with price signals.
I have also heard that I should reward dealers who participate in online forums or write books by steering them business but I'd rather just pay for the books. There is no way to pay for discussion group participation so I share my knowledge in return. When I want a coin it's rare not something that every dealer has a couple of anyway. Expressing moral values though shopping doesn't seem able to provide incentives to change much.
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