Qitzur Shulchan Arukh – 65:18
[The law for] someone who has merchandise that is sold here cheaply and in some other place for more money, whose friend says to him, “Give me this merchandise. I will take it to the place [where it is] more expensive, and I will sell it there and do what I need with that money until a specified time. Then I will reimburse you according to what it is worth over there after I deduce my expenses in handling the merchandise.” If the responsibility [for the merchandise] during the transport lies on the buyer — it is prohibited. But if it is on the seller — it is permitted as long as he gives the buyer something for his effort.
If the responsibility lies on the buyer, then we are back at a case where the seller makes money through waiting at the expense of another Jew. The buyer is buying the merchandise at the time of receipt (which is why their loss would be his), and paying after a delay. If, however, the seller retains ownership and thus risk of loss if something happens to the product, then he is paying someone else to conduct business for him.