That looks a bit more severe than a scratch, but it is still pretty small. It seems the seller is honest enough to have taken a decent photo of it.
A scratch/gouge will tend to cause some lens flare when bright light sources are in or near the image frame.
Lenses, particularly zoom lenses have to 'breath' so most of them have some amount of dust inside them.
How much the lens might be worth depends on the condition of the entire lens, if it comes with the lens hood, the original packaging, and not just the condition of the front lens element.
The condition of the rear lens element is also critical.