Is there a better term for a groceries divider bar?

I think you are referring to the bar that is generally called:

Next-customer bar:

• One of the most used plastic products in supermarkets are "Next Customer" bars.

If Google can be trusted, I would suggest checkout divider as Wiktionary has it. Divider is the most commonly appearing word in all the variant names used by advertising companies and manufacturers that appear in a search: grocery divider, checkout lane divider, lane divider, and so on, but the largest number of image results, for example, come up for checkout divider.

But Google is fickle, so I checked filings with the U.S. Patent & Trademark Office, and here too, checkout divider was the common element. From 1984 we have a patent for the check-out counter divider (#US 4534126 A); thence to the checkout counter order divider, retail checkout divider, and simply checkout divider— plus checkout counter divider bar tracks.

For what it's worth, checkout divider is also offered in translation dictionaries as the English counterpart to séparateurs de caisse, and checkout [lane] divider or grocery divider [bar] for der Warenteiler, der Warentrenner, & der Kundentrenner.