I would say that you're quite right to be cautious.
It could be perfectly innocent in that what you're selling is in short supply and/or that it is worth twice your starting price. But it could be malicious.
My advice would be to continue with the auction as normal. If your item is worth $500 then I'm sure it'll reach that, if not more, once the auction ends. Do not enter into any transaction outside ebay (i think that's contrary to Ebay's Ts and Cs) and wait until the payment has cleared before dispatching the item.
Hope that helps,
AV8