What version of IOS is this iPhone?

I'm selling this on eBay: http://cgi.ebay.com/...00wt_1284… And I got asked that question. I bought it from another person off eBay, and immediately re-sold it after realizing it couldn't be activated with my carrier. So I don't know much about it. Is the answer to that question in the product details, or is it something I should ask the person I bought it from?

The latest update is 5.1.1 I recommend updating to that before you sell it.