So they were doing what everyone else was doing?
It was shunned by the church and jews pushed for slave trading. Similarily you can have society that shuns criminals, although crime still will occasionaly happen, and then have minority of people whose morals have no problem with the given particular crime/vice and will push it on that society and indulge some people who happen to have wealth and lack of morals. That's the 101 of how countries corrupt.
Shunned by the church lmao
Yeah, for example in early medieval Bohemia this was one of the reasons why Bishop of Prague, Saint Wojciech had to run away to Poland and his family was killed. Prague was one of most important centres of slave trading in Central Europe at that time, it was so profitable that local elites reacted reacted very aggresively on any opposition in that matter. Generally slave trade depended on jewish merchants called Radhanites (possibly from the area of Rhone river in France) who traveled areas from Spain to central asia.
Radhanites bought slaves and went either to Venice and from there to Al Andalus, or to the east to central asia (more important direction since most of the silver used in trade was mined and minted in those areas), during the more pagan times local warlords raided their rivals and part of the defeated populations was sold to Jews for silver, this was used to build early states in the area.
Contrary to your retarded fedora tipping, Christianity was one of the main reasons for the decline of slave trade, from both religious (you want to have more converts to save souls) and economic (for more civilised and stable places settling captives was just more profitable) perspectives. Certain areas where it was ignored in this matter (like Prague for a long time) show us that generally with Christianity ignored or not present selling of locals would continue even when it wasn't as profitable, silver from the east dried up in the first half of XI century yet Jews continued to trade even in the 1300s (there are mentions of certain queen or duchess that sold her own property to buy out Christians from Jews).
Without Church slave trade wouldn't be minimalised to mainly foreign pagan elements and locals would be traded normally instead of that being limited to criminal scum and shady shitholes like Venetian Republic, i really doubt that most pagans would gather funds and sold their own property for someone they didn't know and the idea of pagan exchanging himself to save someone not related sounds quite ridiculous yet in Poland monks often did that to save captives from Tatar and Ottoman raids.