Last week Blizzard confirmed that the Brutosaur is going away with the Shadowlands pre-patch. This had a pretty immediate effect as players started scrambling even harder to get the required gold in time.
The expansion is winding down
Prices have steadily been trending downwards for most BfA items for a while. This is expected considering that players are finishing off the difficult parts of the game as raiding, pvp-ranking and m+ mostly winds down.
This will of course continue until l the pre-patch and beyond.
Players are selling to get their Bruto
In addition to this comes the announcement that pre-patch is officially when the Brutosaur will go away. Tons of players are working the Auction house extremely hard to get enough gold for their Brutosaur.
When many players are trying to sell at the same time they will drive down prices. This is happening on many realms right now and prices are hitting new lows.
Vendor shuffles
Some of the players pushing prices down are farmers. In many cases prices are low enough that you can profitably vendor shuffle materials. Tidespray and leather both have items with high enough vendor sell prices that vendor shuffles are possible.
You might also find materials like gems selling for less than their vendor cost.
Stocking up
If you don’t have any short term goldmaking goals the period of stocking up on cheap BfA materials will likely start soon. If you want to stock up I would focus on the materials needed for the two engineering mounts. Both of these recipes are the ones most likely to retain value in the future as crafted mounts always sell well.
So you may want to stock up on ore and some expulsom for your engineer to be ready.
Longer term
We don’t know how inflation will look in Shadowlands. As such I would not suggest investing heavily on non-BfA markets. The risks are going to be too big in most cases, and you are better of sticking to things that make gold in the short term in stead.