eBay recommends that sellers use Promoted Listings to boost sales of products that are already bestsellers. There is a reasonable argument for that, as Promoted Listings are often effective at increasing sales of products that are already selling pretty well.
There is also a good argument that a product already selling well does not need the additional boost, and you will give away precious profit margin when you don’t have to.
The only way to reconcile these two points of view, is to try Promoted Listings for a time so you can calculate the sales and profit you will make, both with and without the additional advertising.
You might have a lower profit per unit with Promoted Listings, but the higher sales velocity could make you more profit in total per week. In that case, it might make sense to give up some margin for more sales and higher profit overall (if you have any margin to spare, of course!)
You will also need to consider your ability to replenish the product when stocks run low. It’s not a good idea to give up margin if the sales volume will be restricted anyway, due to restocking difficulties.