The writer referenced as the inspiration for the precious metals strategy, Andrew Craig (author of "How to Own the World") is the ONLY advisor that I've come across who actually recommends precious metals with the fervour that he does (at least amongst advisors that are otherwise pro-index, FI/Bogleheads oriented and not commission-based/corrupt). But just like you'll always find one or two medical professionals out of a hundred that recommends a different treatment than the other 99, doesn't mean they're right or that you should necessarily follow it. But to be fair, Craig doesn't recommend you go all-in on precious metals, just that you include it in your portfolio. For me, it would be introducing unnecessary complexity, but to each his own.
I do have to say I find it absolutely ridiculous the same dealer who sold you the gold is refusing to buy it back from you though. This sounds like a situation where it might pay off to play the role of the disgruntled Japanese "claimer", demand to speak to the manager and ask why the hell they sold it to you in the first place if they won't stand by their own quality assurances. Pound the counter a couple times for effect.
I agree that this is BS. There might be some backstory, such as 1) they mistakenly bought/sold fake gold or 2) change in ownership, but still BS. This is Japan, and refusing a buyback for precious metal is odd.
So an update on this. I decided to go back to the shop where I originally bought it after trying a few other specialist PM places in the OKachimachi area and asked for a more detailed description of what was going on. Staff were quite willing to explain, it seems that this is stemming from the big refineries in the country who many of these shops sell the PM they buy on to such as Mitsubishi and Ishifuku. As someone else mentioned in the thread, it seems there was a spate of fake bars dumped on the market about 2 years ago and the most common denomination for this was 100g as it was enough to be worth the risk but not too much to draw too much attention. In every case it was non-Japanese refinery bars that were faked. They explained that proof of sale and serial number didn't matter because there were of course cases of those being faked too. After the conversation in their back room about this the younger sales guy mentioned quietly that the pawn shops would not be so worried about this as their aim is to resell the item B2C rather than the B2B route to the refiners. Pawn shop was not something I had considered.
Conclusion is I walked across the road to Daikokuya and sold without hassle, process was negotiate the price (I got them up from 6700 to 6850 per gram), weight test, spectroscopy test, density test then sign a sheet, let them copy my residence card and walk out with a fat stack of cash.