Let's start with a basic bit of sense: If you think the price of something will go down, sell it before it does; if you think it will got up, sell it after it does. If you think it will be stable (or insignificantly different), then sell it now, since gold now is more useful than gold later.
Based on this you probably shouldn't have an inventory in the first place unless you craft in high amount and constantly have mats being mailed in. But we don't always make perfect decisions, so let's just assume that you do have an inventory and not question the reasons why it came to exist.
Prices will go down before the expansion. Gear will lose value as people anticipate it being replaced quickly or at least moving on to the easier task of leveling. This means raiding will go down, bringing with it consumable demand. Crafted gear will be worth less. Together this adds up to a major decrease in material prices.
Prices will go down, but when?
There's the 'natural' decrease caused by materials losing crafting value. But there will be a decrease before that as well, as people anticipate the drop and start selling. You will want to sell before this gets into swing. You cannot just speculate on the real world, you must speculate on the speculators.
If you have any BoEs, get rid of those even sooner. Materials will rise in price eventually as the inevitable parade of alts and min-maxing profession switches come. But gear, that will simply be obsolete. If something will never be desirable again, get rid of it. If it will be, consider how much the price might go up, and consider the bag space it might take. If you're carrying a stack of ore for a few months just to get a 10g profit, you probably lost far more than that by the lost spot for vendor trash. If you have a lot of bank space, then you must consider the negative effect of reduced capital, not that you didn't have to before, but it was a secondary issue to bag space.
For my part, I'm moving down to the minimum. I had tons of gems just sitting around, which I should have sold much sooner. Those are being quickly sold off, but not dumped! They still have value, so it would be stupid to just throw them all out at a fool's price. In particular I'm trying to get rid of my darkmoon trinkets; which is actually bringing in a decent bit of gold. None of this is making me rich, but even if I don't chase gold, I have no objections to having more of it!