Speculation on why few collectors insist on proof of pre-1970 commercial history

I was born in 1967 and don't remember the 1970 debates about the UNESCO antiquities convention. Presumably in 1970 it seemed like a good idea that any antiquities not 'known' before 1970 were fresh finds. Everyone agreed not to buy them, and it was assumed that since everyone would keep their word the tomb robbers, unable to find customers, would take up new professions.

In 1972, Thomas Hoving (director of the Metropolitan Museum of Art in New York) sold a marvelous collection of ancient coins with 19th century provenances to buy a very nice Greek vase called The Euphronios Krater.

Hoving was in control of a gigantic collection of antiquities in 1972 when 99% of known antiquities probably had pre-1970 'provenances'. He wasn't able to resist buying the Krater even though he suspected it looted. He spent $1,000,00 for the dubious Krater, which had been looted, and 36 years later the museum had to give it to the government of Italy.

• The fact that he already had a huge collection of antiquities did not stop him from acquiring new ones.
  
• His high moral fiber did not stop him from acquiring.
  
• The many pre-1970 antiquities available in 1972 did not distract him from his collecting fever.

It's now 2008. No one knows the percentage of ancient coins in collections are pre-1970. Perhaps 25%, perhaps 75%. Only a tiny fraction (Nathan Elkins says 1.15%) of coins for sale claim to be old enough for museums to acquire them—many more are but no one thought to track cheap coins four decades ago.

Hoving couldn't resist when 99% of antiquities were at two years old. It is hard for me to resist when only 1% are declared such. This doesn't make me a bad person, just typical.

I bought five coins this year. Perhaps they were all found pre-1970 but I can't prove it. I see a coin, it's already out of the ground. If I don't buy it the dealer will keep reducing the price until someone buys it.

Even if a coin seems to have a provenance should I trust it? One coin in my collection has as it's provenance “Acquired from C. Holm in December 1974, as a duplicate from the Copenhagen cabinet.”. How am I going to check that? If I cannot easily verify claimed provenance it is difficult to treat claims as being necessities for acquisitions.