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In the context of the baseball card collector example, the legal term that best relates to the buyer's knowledge is unilateral mistake. This concept involves a situation where only one party is operating under a false assumption about a significant fact in a transaction. In this case, if the buyer is unaware of the card's true value or authenticity, and that misinformation leads to a decision to purchase, the buyer may be in a position of a unilateral mistake. This means only the buyer has been misled or confused about the terms of the agreement, thus impacting their ability to consent fully to the contract.

The other options, while they each hold significance in contract law, do not capture the specific scenario of the buyer's knowledge effectively. A mutual misunderstanding would imply that both parties are confused, which is not the case if only the buyer is mistaken. Consent in contract pertains more to the overall agreement between parties rather than knowledge of specific facts. Consensus ad idem refers to the mutual agreement of the parties on the same subject matter, which does not directly address the issue of one party's knowledge being flawed or incomplete.