The standard 2005 HU's do not have the din plug. I don't think the 2004's do either (not 100% sure).
The standard 2000-2003 do have the din plug.

So he's selling a 2005 or maybe 2004 HU.

Guess they found a way to save a few pennies on the HU's by dropping that feature.

As a side note, the stock stereo on my 2005 sounded signifigantly worse than my 2003 X. I couldn't switch out the 05's stereo fast enough.
The 2003's stereo isn't awesome or anything, but it's tollerable to the point that I haven't been itchy to upgrade it.