If you want to pre-order an iPhone 5, you can do so directly through Apple, through a carrier such as Optus, Telstra, Vodafone or Virgin Mobile, or through a third party seller such as Kogan. You can also now do so through eBay sellers with access to Apple's supply chain, but the site has set some strict rules around the process.
eBay's iPhone 5 guidelines allow Apple resellers to list up to 25 iPhone 5 items a week prior to launch, and an unlimited number afterwards. Top-rated sellers can't list more than 4 a week ahead of launch, or 8 a week afterwards. Sellers also need to provide packaging tracking information and comply with eBay's general presales rules. (eBay applied similar rules for the launch of the latest iPad earlier this year.)
eBay's emphasis these days is almost entirely on professional sellers, and the pre-sale rules reflect that. Once the device is actually on sale, I imagine there will be some opportunistic individuals trying to shift newly-purchased devices at inflated prices, even with restrictions in place on how many they can try to sell. Hopefully the range of other alternatives will mean no-one falls for that kind of con.