Nedarim 31B

מתניתין בזבינא מיצעא, ודשמואל בזבינא חריפא.

it must be that the Mishnah is dealing with an average sale, which is neither of particularly low quality and difficult to sell nor of particularly high quality and in high demand. Therefore, when it is sold at the fixed price, it cannot be said that either the buyer or seller benefits. Consequently, the one taking the vow must lower the price when selling to those forbidden by the vow and add to the price when buying from them. And by contrast, the case of Shmuel is referring to a keen sale, in which a sale at the fixed price is considered to be primarily beneficial to the buyer.

תניא כותיה דשמואל: הלוקח כלים מן התגר לשגרן לבית חמיו, ואמר לו: אם מקבלין אותן ממני ⁠— אני נותן לך דמיהם, ואם לאו ⁠— אני נותן לך לפי טובת הנאה שבהן. נאנסו בהליכה ⁠— חייב.

The Talmud comments: It is taught in a baraita in accordance with the opinion of Shmuel that taking an item from the seller to inspect it before purchase is considered like borrowing it. In the case of one who takes utensils from a merchant in order to send them to his father-in-law’s house as a gift and says to the merchant: If they accept them from me I will give you their value, and if they do not want them, I will give you a sum of money according to the value of the financial benefit that I received from them, i.e., I will pay something for the benefit that I received from showing my father-in-law that I want to honor him, then if an accident occurs to the utensils on the way to the house of the father-in-law and they are broken, the buyer is liable to pay because he has the status of a borrower.