Whom do you think would have bought the High Septon’s vestments when the High Sparrow sold them? I mean, what does one DO with such things, since I don’t think there’s much call yet for a museum exhibit of late third century Westerosi clerical finery? The only thing I can think of is that the HS got ripped off, since the only motive to buy those things would be to get the materials (gold, silk, crystals) at a steep discount.

“My lord father gave your predecessor a crown of rare beauty, wrought in crystal and spun gold.”
“And for that gift we honor him in our prayers,” the High Septon said, “but the poor need food in their bellies more than we need gold and crystal on our head. That crown has been sold. So have the others in our vaults, and all our rings, and our robes of cloth-of-gold and cloth-of-silver. Wool will keep a man as warm. That is why the Seven gave us sheep.” (Cersei VI, AFFC)

The cloth-of-gold and cloth-of-silver would sell like any other fabric; prior to the advent of mass-produced cloth, it was quite common to break down old clothing for “parts” and for those parts to be sold without much of a markdown. Silk is silk, after all…

Ditto, the rings would seem to be pretty fungible in the jewelry market. I think you’re correct that the crowns would be a little bit more of a niche product, but there have to be enough wealthy devout people around looking for a relic. And at the end of the day, just having them around was a decent way of storing wealth.