All sellers have to specify a dispatch time (1 ,2, 3 days etc) on the listing.
There is also an expected delivery date for each item shown on your purchase history.
You can start an 'item not received' claim with eBay once the expected delivery date has passed. They then have 5 days to provide proof of dispatch after that you can pass the claim on to eBay customer service.
I would just wait until the minimum time has passed and then put in a claim rather than contacting them. if you paid by PayPal then eBay/PayPal take the money you paid out of their account and put it 'on hold' until the claim is resolved.
One point to remember is that a lot of people post the item without listing it as 'dispatched'.