The listing specifies it comes with one long Waterman Blue cart and "5 extra standard ink cartridges (Royal Blue) included". Most brands of standard carts are sold in boxes of 6. So probably someone bought one package of carts, put one in the pen to test it out or use it, and then didn't end up using it. So they let it dry out in the pen, and it sat un-flushed for probably 10+ years.
I wonder what brand of cartridges it could be; Pelikan "Royal Blue" cartridges do not fit in the barrel. Possibly they installed a cart into the section, enough to get some ink into it, and found the barrel couldn't be fit with that cartridge. In disgust they tossed the cartridge and never used (nor flushed) the pen.
Sadly I seem to like making up the hypothetical stories behind such listings.
We already know there is a corrosion problem with the nibs on these pens (and the trim and/nib of a few other similar vintage Waterman models seems to be frequently corroded in auction listings) when not flushed very well before periods of dis-use. It is one of the few downsides to the pen. I wonder if it is still a problem with their current production materials on current models or not.