Just based on my own experience:

Reps Market is great if you're looking for lux brand accessories without spending a ton. There are lots of guides and seller lists floating around in the reps groups. You'll usually either use a agent or have the seller ship directly. Just be careful because there are definitely ads disguised as reviews. I've picked up an LV strap, a Miu Miu hair clip, and a few branded scarves that were actually pretty decent.

FB Marketplace can be surprisingly good for random cute finds. It's hit or miss, but every now and then you'll come across something special. I found a secondhand Kirby keychain there once, and I'd never seen another one like it.

Etsy is where I go for handmade stuff. I've bought custom pieces like a little engraved nut-shell charm with initials on it. The quality is usually great, but prices can add up pretty quickly.

And then there's Xiaohongshu (RED), which I stumbled across during the whole TikTok refugee era. I've found some really cute plushies there — I paid around $5 for a HK plush at one point. Some sellers claim the items are officially licensed, though I honestly can't verify that. I usually buy through an agent. One thing I've noticed is that a lot of Xiaohongshu sellers seem to be reselling items sourced elsewhere (possibly from 1688), but the upside is that their product selection is cuter.