That's nowadays, back then they'd auction the items and resell at whole price depending on the economy
you're kind of saying the same thing... wholesale price is less than store bought. Auctioning works for pieces with some kind of intrinsic value in them like the artist/designer, who they belonged to, notoriety of the main stone etc...
Honestly it would sell even higher at an auction though. It was owned by the Duchess of Taran, which puts those jewels at a higher value for an auction.
Actually no, save few exceptions, like maybe something crafted by a master jeweler, You can never recoup 100% of what was spent on a piece of jewelry. I think it's like when buying it jewelers will evaluate it as the sum of its parts, not the piece as a whole. The craftsmanship is lost, again unless it's some artist or similar