What I think is interesting about the word flea market is that it's a calque in pretty much all languages.

The Swedish word is "loppis", which is a cutesy colloquial term for "loppmarknad." Loppa, meaning flea, and marknad meaning market.
Flohmarkt in German also means lit. "flea market."
Marche aux puces is French, where "puce" means flea, I think this might be the origin of the term.
Japanese has the casual term フリマ (fleama), short for フリーマーケット, which is just the English term "flea market", there's also the term 蚤の市, just meaning "market of fleas."

I believe Portuguese calls it a "thieves' market", but Spanish, Italian, Russian, Turkish, Dutch, and Mandarin all use their own native words for "flea market"; mercado de pulgas, mercato delle pulci, Блошиный рынок, Bit Pazarı, Vlooienmarkt, 跳蚤市场.

For all of the concepts and such that are identical across cultures, few things have universal names. Typically they enter the language as loanwords as well (e.g. karaoke, from Japanese '空オケ', hollow orchestra), so the term "flea market" stands out to me. I'm sure there are lots of other similar things I'm not aware of though.

Edit: It's worth mentioning that other than Swedish (native), English, and Japanese, I don't speak any of the other languages. I've asked a Russian-American friend about the Russian term, and a friend in Taiwan about the Mandarin term. Otherwise I've checked dictionaries and the like. Don't take my word as fact, I'm not a linguist. It was just a pattern I found interesting, because the term itself is so particular. Any and all corrections are more than welcome.

I'm also delighted by the discussion this has sparked! 💖