Could I please ask for opinions on whether sellers find this marketing effective?
Thanks in advance.
We have been doing them this time since last summer, around 10% cost with a little variation.
The advert only becomes live for a particular item when you have the buy box, so if you have it only 20% of the time then you’ll not be paying for clicks the other 80%.
Price per click almost never approaches our maximum, often less than half.
Keywords are an issue, I’m often going in and altering keywords and updating what a campaign covers, pretty sure I’m figuring out the best keywords just when last item of that type is sold. 
Seriously, try it with a few things and keep an eye on it, cost you a few pence to a few pounds depending how big and how long you want it to run.
Some campaigns and ads may do well, some not so well - so you can end up tinkering with words.
Suprising how much old stuff, over 3 months old, that got rid of late summer and through the autumn last year.