Well i doubt there would be betrayers among Argonians because they are very LOYAL to each other, but i think that the Dunmer, they go into Black Marsh and take slaves by force, because Dunmer are more racist than skinheads, not only that, but they are like the High Elves, who think that everyone is inferior compared to them
Atcually, Argonian Tribes might think it good buisness to sell members of enemy tribes to the Dunmer. The tribes could trade their captured enemies for food, drakes, or exotic items not present in Black Marsh. Tribe A hates Tribe B, so why shouldn't Tribe A sell Tribe B to the Dunmer?
Also, taking slaves by force is hard work. You risk lives, equipment, and in general take more time raiding settlements for slaves. It's much easier to buy them from local warlords, tribes, etc. Then you only have to pay for their transportation and the cost of purchasing, which is probably cheaper then a raid.