There is more to these scenarios than intervening on an "uninformed" sellers behalf. I believe the value of collectibles really belongs to the one that held it over the years. Heirs in most cases inherit an unearned windfall but (I agree) it still "belongs" to them. However, where is the responcibility of the grantor to make the value known, or it finally rests with the heir to exercise due diligence to find out. If they are negligent then good samaritans can't always save them. Remember "caveat emptor"? Buyer are told to beware, well sellers also need to beware.
The only reason prices on collectibles keeps climbing is people with more money than others want them. The sandcast case is inferior to the pressure molded cases but people with more discretionary money want one so they become more valuable as an investment.
I would like to score one cheap, not for the resale value, just because I'm sucked in to the hype too! I could die having owned a "sandcast" and if they fell out of vogue & my heirs couldn't capitalize on that, well, they didn't earn it.