Inflation is always a factor in World of Warcraft as more gold is being generated from vendors and mob drops every day. If we look at item prices in Classic we can easily tell that inflation has really started going off lately.
Gold in > gold out
There is more gold coming into WoW classic than there is going out. After the initial big purchases for spells and mounts there really is very little gold going out of the game. You spend a bit on repairs and potentially respecs of course. Outside of that there is nothing to spend gold on that is not generated by other players.
Now that the vast majority of players are max level and have their mounts the amount of gold going out of the game has really dried up and prices start rising more quickly.
Mage alts galore
If you look at any “How do I make gold” thread on the classicwow subreddit one of the most common answers is “I leveled a mage alt for farming”. Mages can generate ridiculous amounts of raw gold from their farming methods and they contribute heavily to the inflation. The current content lull which is the longest period without a new difficult raid is also a period where a lot of players will likely be stocking up for the rest of classic. This is the last breather before the two potentially difficult raids in classic.
Illustrating the inflation
Arcane Crystals are a nice item to look at this effect. The amount needed has not changed since Blackwing Lair launched back in February, so any price changes since the initial Thunderfury Rush will likely be caused by inflation in the gold supply. As we see below the price on my realm has trended upwards for a while. You can also see the green line representing the region price following a very similar curve.
What do you do about it?
There’s nothing to do. Just keep doing things that make sense in terms of profit. If you are planning a break I would invest in Arcane crystals or Black Lotuses to keep up with inflation rather than just sitting on a pile of liquid gold. Inflation has always been the case and will generally favor gold makers as we reinvest continuosusly so when prices go up we make a profit.