I just want to know what to do if I come across these. I see some herb vendors also have some mats. Is it worth the price for someone who will be doing professions together with leveling?

If your goal is to get epic mount as fast as possible - No
If your goal is to get your professions up as fast as possible - Yes
If they are really expensive on AH - Yes
If they used for an enchant people want that covers cost - Yes

Get an add-on like auctioneer or manually check the AH price if you have an alt near the AH. Even if you aren't levelling that profession, you can likely by those limited quantity items and AH them for profit.

The lesser magic essence in your screenshot example ;D imo yes , if you are a wand user with enchanting + tailoring and you want to rush for the wand. the chances to get lesser magic essence by DE the first tailoring gear is very low. to have the lvl 5 lesser magic wand from early on is huge.

but for all other stuff, not really. I only know the devilsaur patterns in un goro been worth to buy. or the bronze tube for the duskwood q.
And the winterspring mooncloth pattern + the runecloth bag patterns aswell (both from the same npc in everlook).

But the pure mats , I don't think so.

Save your money you can disenchant items for that stuff if you are doing enchanting. Just do tailoring with enchanting and you can make stuff to disenchant. Vendors are money stealers and only good to sell things too and that's about it.

There are some profession recipes that are sold by vendors only, those can be sold quite profitable if you manage to camp a certain vendor. And no, I will not state which ones I mean openly, you can look them up :P