If it's a dual, then it's a dual processor. If it's liquid cooled it would either be a 2.5 or 2.7Ghz DP.
The later G5s (2.0DC (Dual Core), 2.3DC and 2.5 Quad) all have this plastic strip on the back left. In these photos that strip is not present which means it's not a dual core.
The Late 2005s are also distinguishable by the larger front fan assembly and the single, large "G5" block on the processor, as opposed to two stacked "G5"s on earlier models.
[doublepost=1563158531][/doublepost]OP, see if you can get the seller to get a closer picture of the long gray sticker on the side of the computer, where the door would block it. It's slightly visible in the second shot where the whole thing is upside down, towards the top of the photo.
That would present the make and model, + tech specs.
As to the video issue... If you do end up getting it, try removing the GPU and cleaning its contacts, then reinsert it and try again. Could be a bad connection.