Were the Martinez brothers planning to sell the women or overdose them?

Sell as obedient slaves by hooking on drugs. That will be a thing in selling I think with cost. Although the slavers sold to may get drugs at a discount. Just like in Taken the bunch really don't know the amount to give and some die. Just like back in slavery days they really crowded the ships and then more room later as some died off.In this you grab more girls than you need and some will die off later.

Overdose may be just shit application of a drug by what clearly is not a professional or even a suicide by the girl to stop being raped every day.

Slaves controlled by drugs is what actually happens.