I recently learned this, myself. Comparisons of slavery with serfdom are usually dismissed (understandably) as comparing apples and oranges, minimizing the harshness and depravity of chattel slavery in the Americas, and even interpreted as an attempt to minimize or at least distract from European and European-American culpability in the American slave trade by appropriating victimhood status. But in parts of the Russian Empire (in Europe, not Asia--though perhaps Asia, too?) there were serf markets well into the 19th century, notwithstanding laws that nominally prohibited the sale of serfs. Many immigrants to the Americas were surely not far removed from those circumstances, if not having fled them directly. So just on a purely factual basis, there indeed was chattel slavery in Europe of Europeans, ending not long before chattel slavery in the Americas--1888 in Brazil. That fact doesn't refute the latter criticisms, though. (And it also glosses over the fact that Slavic peoples weren't really considered "white" by most Western Europeans. Heck, even today that distinction lingers, detectable in the sentiments undergirding Brexit, e.g. in resentment of Polish laborers.)

Somewhat related, chattel slavery remained a thing in the Ottoman Empire well into the 20th century, albeit substantially diminished; and not completely outlawed until mid century. And I guess this is where a lot of white slavery hysteria in the U.S. originated[1], as in the Middle East owning Circassian slaves had long been a status symbol, so much so that over the centuries they significantly effected the genetics of ruling elite families through their popularity in harems. Circassians were literally the archetype for "white" in Hitler's Aryan racial system, so the trade in Circassians represented the epitome of white slavery. Though, perhaps the real horror regarding the Circassians is the Circassian Genocide at the hands of the Russians: https://en.wikipedia.org/wiki/Circassian_genocide Sadly, when you delve into the history of ethnic cleaning by Russians, there are sadly many more stories like that.

[1] As opposed to the capture and sale of white Americans and Europeans by North African pirates. That's where I used to think the hysteria came from, but reading about the history of the Circassians, I suspect that's the real origin, or at least the more important dimension.