I'm wary with second hand as it's a rather large market and I feel you have to really know what you're buying. I know the pieces I own are 24K gold but it's from a very specific region so wouldn't feel confident spotting it at a second hand seller if they claimed something was 24k. There also seems to be a large variation in quality, our finest independent jeweler has lovely quality things with price tags to match but they also sell second hand pieces not of their design. It's just good business sense, people can buy something at the place with the best reputation at all budgets. I often walk past the second hand window display and you can definitely tell there's a quality downgrade, stones aren't as nice, the metals have a different reflection, etc. and it's not down to age. They're also designs that I'm not particularly attracted to, very safe, proportions aren't as nice, etc. It seems fairly priced but I suspect outside such an established shop it would be cheaper.
I sometimes nose on a French high end second hand seller's shop online that has in house experts (I usually keep an eye on the designer bags, the 'lower' end stuff not the Chanel and Hermes) and they have some beautiful vintage pieces that are really unique, and would very much suit the rest of my rather eclectic inherited collection as it's of similar eras. But if you look at the prices, you'll understand why I thought Elle's Cartier necklace seemed very reasonable, I just assume the prices are reasonable in comparison to the same level of craftsmanship new but I really have no idea.
Xing, didn't you mention a sister (or SIL) that worked in jewelry picking up good bargains with all that knowledge?
Certainly make for nicer souvenirs, quirky art and jewellery was usually what got schlepped back, I promise they didn't fob the locals off with beads. When you travel to specific places they also often have mines and tend to be quite a bit cheaper because of that. I have some silver and I think lapis lazuli pieces from South America when we traveled there, it was very cheap, even on my teenage budget. I just don't wear it much, I liked silver then but prefer gold now (thought it was very fuddy duddy), luckily I can wear both, only rose gold looks weird on me.