OK, our "Update All" feature was short-lived. The button is gone now.
Instead, we are going to show buyers 2 dates on all product postings - Date when product was last updated, followed by the Date when Seller was last login.
Btw, on your own product postings, once you login, you'll also see a Date when you posted that product. We decided to drop this date from the page meant for buyers, as it's not as relevant to them when product was posted, as much as when it was last updated. Plus, 3 dates displayed one after another looked like it was too much info to absorb.
On products, whose owners didn't login in more than 6 months, you'll see a message like this: