I’m close to pulling the trigger on purchasing a used Sony 24-70mm gm lens. It’s about $300 cheaper than others and the only flaw is a small scratch on the very outer edge of the lens (as seen in photo). 
I’m new to the camera world and wondering if there’s a test or method to check if the scratch will impact video quality. Even if there’s a small reduction in quality, I wouldn’t go with the purchase. My untrained eye can’t tell if the contrast or clarity of the image is impacted by looking through the monitor. Any thoughts appreciated.

A scratch on the front element will not be visible in the images. It will be out of focus and blurred out of existence.

There is a slight risk of introducing a flare, and there is a standard way to fix that which will sound silly. You fill the scratch with black ink! I don't know if it still exists, but the recommendation used to be to use "india ink". You want the ink to fill the scratch, but not overflow, so a thin ink is better. If you don't want to try the ink, then shooting as-is should be fine.

Not sure that $300 is enough discount, though, because reselling the lens with the scratch will be more difficult. I'd be inclined to push for a bit more of a discount...

I agree.  That's nothing.  Take a black SHARPIE and dab it -- to avoid possible bright spot.  If $300 is a 25% discount (or more), I'd go for it.  With the savings, buy a multi-coated UV filter to avoid more scratches.

Edited October 16, 20232 yr by XKAES

Thank you for the advice. If there’s only a remote risk of lens flare (and I can fix that with ink) then it might be worth the $700 US they’re asking. At resale, I would mark the price down similarly.

Yes, the risk is very remoted and only in specific lighting conditions.

A few years ago I took some pictures with a cracked polarizing filter. I did not realise immediately because the filter cracked by itself from side to side, without being smashed against something. When I realised, I took the filter out and I shot the same pictures without the filter. When I looked at the pictures, I saw no difference except in the one that was shot with the sun directly into the picture.

See Roger Cicala of Lensrentals who has dealt with 1000's of lenses post on Front Element Scratches. I think the lens you are considering will be just fine as far as that scratch goes providing it did not result in mis-alignment of the front element.

He also has a practical guide How to Test a Lens.