It's not a one time transaction. If they buy slaves from your rivals, they have more money to spend on arms and a need for more people to sell. So refusing wasn't a safe option, creating a larger demand changed the game dramatically.
>In a nutshell, Europeans didn't turn that people into chattel slaves, they where turned into that by the other natives that enslaved them.
You could sell a debt slave with the thought the new owner would abide by the same rules. Or if slaves were treated as members of your household with an free children and a general path back to freedom, you might expect the buyer does the same. If you don't view people as property, you won't expect what someone buying them will do.
>Yes, the 10% that only occurred for a short time compared to the rest of the all Europe and because you keep accounting for people passing by than only spent there some weeks (i.e. the slaves waiting to be sold).
Source? As given how poorly Portugal looks in the slave trade, the last ship of slaves being Portugese to Brazil, that is hard to believe. And are you really happy with the argument "Portugal shouldn't overly acknowledge the slave trade, they barely kept any in the mainland while profiting off their sale and work for centuries."