You need to refer according to
Hector12 initial statement
Warring European nations never sold their enemies to Arab slave traders.
A) The Byzantines were not Arabs - but Greek-Roman Europeans.
The Vikings or aka the Rus - never conducted organized slave raids or sold their own Rus or a member of their Odin faith to anyone.
They made war and raids onto everyone - thus capturing people (the warriors they killed) whom they then used as slaves, the more correct term would be serfs.
That e.g. Slavic serfs or a serf originating from e.g. Spain due to a raid, were sold to other bidders - possibly also to Arabs or Turkic people certainly can't be out ruled.
B) The term "European Nations" implies the 10th century onward - therefore no more Vikings. Since these "European Nations" were all Christian Nations one can be very sure that no Europeans were sold as slaves or serfs to Muslims. or that any European nation conducted slave raids onto another European country so as to sell slaves to Arabs or anyone else.
This doesn't exclude the possibility of some e.g. Sardinian Pirates (which wouldn't be a European nation, nor acting on behalf the authority of a European Nation ) - capturing e.g. European women and selling them to some Muslim Harem manager.