The advice I give to students is that if strapped for time, there are three concepts to know well for Secured Transactions on the bar exam: attachment, perfection, and priorities as they relate to creditors. But if less strapped, I'd take some time to understand how buyers might work their way into the mix.
The relevant question here is whether a buyer might have better rights to the collateral than a creditor who has perfected a security interest in that collateral even if perfected before the buyer purchased the collateral. The key distinction here is between buyers in the ordinary course and buyers not in the ordinary course. A buyer in the ordinary course is one who buys goods in good faith, without knowledge that the sale violates the rights of another person. Further, the buyer must buy the goods in the ordinary course of business from a seller in the business of selling goods of the kind purchased.
The significance of satisfying that definition for buyer in the ordinary course is that such a buyer takes free of any non-possessory security interests in the goods created by the buyer's seller. Importantly, this is true even if that security interest has been perfected and even if the buyer knows of the security interest (as opposed to knowing that the sale violates a security agreement).
Things go a bit differently if the buyer is not in the ordinary course. A buyer not in the ordinary course (a buyer who does not satisfy the definition as stated above) take subject to perfected security interests. In contrast, such buyers take free of unperfected security interests unless they know of the security interest when they give value or take delivery of the collateral.
There's an important exception for buyers not in the ordinary course that has appeared on the exam from time to time. Consumer goods are goods used or bought primarily for personal, family, or household purposes. In the case of consumer goods, a buyer (even if not in the ordinary course) takes free of a perfected security interest if the buyer without knowledge of the security interest buys for value consumer goods before a financing statement covering the consumer goods has been filed. This is called a consumer-to-consumer sale because for this rule to apply, the goods must be consumer goods in the hands of both the buyer and the seller.
An interesting point to note here is that another rule states that a PMSI in consumer goods is perfected as soon as it attaches. This rule here regarding consumer-to-consumer sales incentivizes creditors to file a financing statement even though there may be automatic perfection. By filing the statement, there's less risk that a buyer not in the ordinary course will have priority over the creditor.