If the bracelet is matched to the watch I am in favor of keeping them together. However it would probably not be cost effective to buy a bracelet just to sell with the watch.
What looks better on the watch?
If the braclet band is in good to excellent condition, leave it. If the braclet has the snot taken out of it, put a strap on it & toss it in with deal.
If the braclet has excessive wear, toss it to the side & if the buyer inquires, send it to them, use at your own risk.
If it is gold & worn, leather strap & hock the gold.
I think it also depends on a watch. With the Speedmaster both work well. But, with my Seamaster or LeGant chronographs where they lack dark contrasting bezel the metallic bracelet just makes everything too bland. I love straps!
I like to have a variety in my collection, so if you have a time appropriate bracelet then why not show it off. It gives a potential buyer the opportunity to see how it may look (even if it’s not included in the sale).
My personal preference is not to put a bracelet on a stark silver dial as it lacks contrast, however if there is a little colour like below it will look quite pleasing. The second picture is silver so I prefer a dark brown alligator strap.