This is actually very common; pages removed using the Google Search Console tool, are not not meant to be permanent. Google even lists this under the
Remove URLs tool section:
To remove content permanently, you must remove or update the source
page. Temporarily remove URLs that you own from search results.
The difficult part of what you are trying to do is de-index a page already indexed by Google. Once Google indexes a page, it doesn't like to let it go, even if you
410 the page, it will periodically over time come back and hit it just to see if the page has returned.
As long as you have the correct
noindex tags set, you should be fine. Over time Google and other Search Engines will de-index the page -- but the key here is time. You can combined the
noindex tags with the GSC
Remove URLs tool to have a more immediate effect.
What I would do is: go under the
robots.txt Tester section in the Google Search Console, and test one of the URLs you are trying to de-index from Google (such as your
example.com/buy). Assuming there are no conflicts, move on, if there are conflicts, address them with your
robots.txt file, as Google should be able to crawl the pages you are trying to de-index.
After confirming your
robots.txt is fine, I would then move on to making sure that I have either one or both the
meta (preferred) or
header tag configured properly on the pages you want de-indexed:
Using the robots meta tag (preferred)
<meta name="robots" content="noindex, nofollow">
Using the X-Robots-Tag HTTP header
X-Robots-Tag: noindex, nofollow
More info on these can be found here.
If all this is done, you should eventually see the pages permanently removed from the Search Engine's Indexes.