Bernal Díaz's text definitely has "contrataban" here; see, for example, Guillermo Serés's edition:
https://books.google.es/books?id=_Ly7CwAAQBAJ&pg=PT1158&lpg=..."Contrar" is simply a spelling mistake in the question term.
"Contratar" meant "Comerciar, traficar, ajustar, convenir, ò hacer algun contrato ù obligacion" (Autoridades, 1729).
Patricia's first answer, "traded", is correct, though "bartered" would be more accurate (barter is a form of trade). It says "contrataban CON ellos", meaning that they used them to trade with, to buy things with. It doesn't mean buying slaves and it's not a synonym of "vendían": it doesn't mean selling them for money. It means what Bernal explains in the next sentence: "trocándolos a oro y mantas y cacao": exchanging them for goods.
With 10 agrees already I think it would be a waste of time posting another answer, but that is what it means.
peer agreement (net): +11