I have been looking for a used guitar here on Ebay and I found a great website that lists most manufacturers serial number sequences so that you can get an idea of when and where any guitar you might be looking at was made.  If the guitar listing you are looking at does not show or list the serial number feel free to ask the seller for this, it is for identification purposes and should never be a problem for the seller to give it to you.  The site is called guitarnucleus.com, I cannot link you to the page directly here but if you go to their main page and look at the bottom for the link to 'serials' they list all the main manufacturers. Happy shopping!