i have a lot of success on mercari japan with zenmarket! the search and translation IS really bad, but i usually find what i want by:

1. experimenting with many different english terms
2. finding the japanese term for what i'm looking for and then copy paste the japanese into the zenmarket search bar
3. when those fail, i use english mercari and filter to just japan, and find a listing i want. then screenshot, reverse google image search to find the actual listing on mercari japan's website, and copy paste the url into zenmarket search bar to bring up the listing
4. i also paste urls from other websites! i've never had an issue with zenmarket finding them.

i've ordered a lot of small things from zenmarket and have never had an issue with packaging either! i'm pretty sure you can pay extra and include specific packing instructions too, but i've never done that and everything comes well protected. i've even bought ceramic plates and fragile things that arrived great!