What is the ruling on taking merchandise or bargaining over its price when there is doubt about the seller's consent, and in these cases, is it based on consent or lack thereof?

There is nothing in what the questioner mentioned that contradicts the conditional consent required for the validity of a sale. What is meant by consent is choice and the absence of coercion. Consent is an internal matter whose ruling is tied to outward expressions like the صيغة (offer and acceptance). The hadith "The wealth of a Muslim is not lawful except with his willing consent" is more pertinent to gifts and donations than to contracts of exchange like sales.