Is there any advantage to buying his new brews at full price when they first come out vs. waiting for them to deflate and buying them from other users? Like maybe there'll be an achievement or something?

So far there isn't. The only bonus thing associated with his brew club so far were the 3 bonus items we got for signing up during Alegarten. I suppose there may be achievements added later for feeding all of his brews to one of your pets, but that's just speculation and not a guarantee.

Instant gratification!
Of course there is the risk they will inflate rather than deflate, due to limited supply, once they retire each month. Since people do feed them to their pets, there might not be many floating around to buy.

I would ignore what has said about retaining or gaining value.

The Selbstgebraute was his first brew. It was selling at 50KsP at the end of its month, and is now selling for 27KsP. Me, I buy a few copies of the new month's offer just to make sure I've got one, and then I wait until the end of the month to buy more at a massive discount of them if I think I might want them.

I have no idea why folks are selling them for so much less than what they paid or even any amount less at all, and I did ask around to and got a blank so ??? as to why this is the way that it is.

During the Councilor's cruise sitewide event, the ship had a posh restaurant with exclusive food items in it much like there were food trucks at Peka Glade. For many years these items sold for less than what you paid for them too. By the time you get your investment back on these, you could've used that sP you originally laid out to restock-sell-restock-sell regular items many times over.

Wow do I feel dumb! 25k??? And here I paid full boat for them 'cause Suds has to have them all in his TC, and I assumed they would cost me so much more in the user shops... (Sneaks away to hide in embarrassment) Still, I suppose I can still ride the instant gratification boat... all the way to the bottom of the lake....