Supermarket etiquette can vary depending on cultural norms and individual preferences, but generally, it is considered polite for the shopper who just finished unloading their groceries to put the divider bar down. This practice helps the cashier and other shoppers clearly distinguish where one person's groceries end and the next person's groceries begin.
By taking the initiative to place the divider bar, the previous shopper signals that they are done unloading their items, and it indicates to the next shopper that they can start placing their groceries on the conveyor belt. It is a small courtesy that can help maintain a smooth and efficient checkout process.
However, there is no strict rule on this, and sometimes, the next shopper in line may offer to place the divider bar as a gesture of kindness. Ultimately, it's about being considerate and helping each other in a shared public space like a supermarket checkout line. If you find yourself in either position, putting down the divider bar or waiting for the previous shopper to do so, just be mindful of others and act accordingly.