There are two important provisions in UCC 9 (Secured Transactions) that deal with buyers. Both provide a buyer with protection even against those who have perfected a security interest. But along with that similarity, there are some material differences to keep in mind.
A buyer in the ordinary course of business is a person who, in good faith, and without knowledge that a sale violates the rights of a third party, buys goods in the ordinary course from a seller who is in the businesses of selling goods of that kind.
And being a buyer in the ordinary course comes with some privileges. With exception for a person buying farm products from someone in the farming business, a buyer in the ordinary course takes free of a security interest created by the buyer's seller. And, importantly, this is true even if the seller's security interest is perfected, and even if the buyer knows of the existence of that security interest.
In addition to protection for buyers in the ordinary course, there are also protections for buyers who purchase consumer goods. Buyers of consumer goods are buyers who buy goods from a person who used or bought the goods for use primarily for personal, family, or household purposes.
Just as with the buyers in the ordinary course, buyers of consumer goods are privileged with some protections, though the requirements for those protections differ. For a buyer of consumer goods to take free of a security interest (even if that security has been perfected) the buyer must buy the consumer goods without knowledge of the security interest, and must buy the goods for value. Further, the buyer must buy the goods primarily for the buyer's personal, family, or household purposes, and must buy the goods before there has been the filing of a financing statement covering those goods.
That's to say, if a seller took the step of filing a financing statement to perfect a security interest, then that will prevent a buyer of consumer goods from having better rights in those goods than the seller. This is one reason why it can benefit a seller to file a financing statement even if the code allows, for example, for automatic perfection: doing so is an additional layer of protection.
Also, an important distinction to keep in mind here is that with a buyer in the ordinary course, the buyer will have better rights than a perfected seller even if the buyer knows of the perfected security interest. In contrast, with a buyer of consumer goods, for the buyer to benefit, the buyer must be without knowledge of the security interest.