Yes, you actually need to find another way here:
Writing you own solution for parsing the wowhead isn’t the best choice here, but it’s also an option. And of course wowhead doesn’t want to share their data with you, or anyone else. They doesn’t provide an API and their official position is:
We don’t won’t to be webscrapped
Sas148#0001 @Discord
Btw, you could ask more from @Erorus, he is part of wowhead team since 2013.
So what to do now?
You may already notice (link, if you don’t) that I have questioned the same thing couple of week ago.
So, long time ago (in the Community endpoint and Mashary era) the item endpoint does return the isAuctionable: Boolean property. Now, it doesn’t.
So, you could try Erorus’s newswire API, https://newswire.theunderminejournal.com/phpMyAdmin/ you’ll be very interested in tblDBCItem table and it’s isAuctionable column.
If you want to check if item not just have price from vendor property, but actually sold by vendor itself. Well, wow.tools and CASC Toolset (for ripping the game tables) should become your best friends.
I already noticed, that you are looking for the same data, as I do, so I guess I could predict that your next question will be relevant with reagent_items field, and how do determine which item is a reagent, and which one is derivative from
By the way, you should try to join Battle.net API Discord. Link to this discord community you could find in relative thread. Guess, it would be helpful for you.