In theory, it’s correct to say “the person with whom I traded”, but that is extremely formal in today’s English. I real life, we usually say “the person who I traded with”. I often say, “the person whom I traded with”, but frankly, that’s a bit unusual. Note that the rule against ending a sentence in a preposition is a myth (although some people do believe it). Note that in your example, “who” can be replaced by “that” (since it’s a non-defining clause) or even omitted altogether (since it’s the object, not the subject). By the way, do not omit “with” — the slave trade is illegal and immoral.

Who vs Whom can be tricky to use. In this sentence, you would say "the person with whom I traded." Who is used when you are talking about the subject of the sentence. For example, you could ask: Who ate my sandwich? You wouldn't say "Whom ate my sandwich?" since that is incorrect. Whom is used when you are talking about the object or preposition of the sentence. For example, you could say "With whom did you speak to?" To check if you need to use "who" or "whom" in a sentence, use the following trick: Ask yourself if you can replace the word with "he" or "she". If you can, then use "who". If you can replace it with "him" or "her", then use "whom." I hope this helps you.

"with whom I traded" would be correct, but it is very formal and legal. I would say "the person whom I traded with" because "who" is not usable here.

"The person with whom I traded." I only use that form if I'm writing a legal document or something formal though. In colloquial speech it isn't that common. People might even say, "the person I traded with."