I was watching a history video where the host claims that Franks and other western Europeans sold Slavs and other North-East Europeans as slaves too the Muslims, but i always thought that with the rise of the Catholic Church and Feudalism, slavery vanished in the West with the only real exception being border states that were influenced by the Sareceans like the Iberian Christian states or Sicily, which wouldn't have access to North-East European peoples, so were these slaves sold by the Vikings, did the Franks make an exception for non Christian Europeans, was it a black market or What?
The best general source I know of on this (though there may be more narrowly focused and detailed works on the Franks) is still Mike McCormick’s 2001 “Origins of the European Economy.” You can find a review here: https://eh.net/book_reviews/origins-of-the-european-economy-communications-and-commerce-ad-300-900/
Part of McCormick’s argument is that the highest value commodity that, at this point, economically starved Europe could export in order to bring in gold, silver, and other goods was slaves. Some of these were coming from Northern Europe and making there way our via the Scandinavian settlements in what would become Russia. Others were shipped our from Central/Western Europe and from the eastern border zones of those coalescing political entities. Venice actually got its start as a slaving port and the slave trade fueled Venice’s economy for generations.