What you're proposing isn't inherently a bad idea, but it is something that already exists (i.e. what you're talking about is called a fulfilment service, and there are a number of these in WA already).
The kinds of sellers who will either establish distribution centres, or use third party fulfilment services, in WA are typically those that can almost guarantee it will be cost-effective to do so. This means their product is likely to be in high enough demand (possibly in relation to somewhat time-sensitive items) to warrant the extra expense, or they are larger, bulkier items where it's cheaper to transport in bulk and store them there, plus ship out from that location, rather than ship out from any of their other interstate locations. For you, that would mean either offering something the more established fulfilment centres don't already, and / or a lot of storage space. T
Many businesses like I describe don't seem to find it a cost effective venture (if just going by the number of sellers I've bough from who have multiple distribution points), mostly because while it caters to buyers in WA, it is unlikely to cater to buyers in other states, meaning the stock that is held in WA is almost exclusively reserved for WA buyers, who are also likely to be lower volume buyers (this is generally why most stick to fulfilment centres in Sydney, Melbourne and Brisbane). I also suspect larger sellers are less inclined to have a portion of stock they can only move to buyers in one state, and likewise smaller sellers can't afford to split their stock in order to get some purchases to WA buyers a bit faster.
For a seller, if it adds a couple of dollars overall to the cost of getting a package to a WA resident, they could well reason (if they get a decent amount of WA buyers indicating they want things faster) that they are better off adding a small amount (like 50c) extra to their pricing and shipping everything to WA via express (I know of some sellers who in fact do this, and also some Perth-based sellers who chose to ship everything express as well).
I'd suggest taking a look at what some of the larger fulfilment centres over there offer, and seeing if there may still be a niche in the market you'd be able to fill (they certainly aren't "cheap" - at least not to me - but they tend to cater to a range of business sizes already, and offer a lot of guarantees with regards to dispatch time, and order / inventory accuracy).
