I myself am considering one (non bracelet).
I imagine that such watches are hard to get new because they are of extremely high value, and not as many are made by the manufacture (which is fair to prevent an oversupply). From my understanding, this piece is not an application piece as my AD has had a few 5204's come by in the last 2 years. Is the resale by a previous owner or grey dealer? If it is a previous owner, maybe he had regrets on buying a high value piece, needs cash urgently, or is raising funds to buy an even higher value piece. I don't recommend grey dealers because likely it means you will be paying a premium on a watch you generally should be able to get.
The watch itself is stunning, particularly the movement side. In the end the most important question is whether you love the watch, how you like the bracelet, and whether it fits nice on your wrist. It is a matter of personal taste. I prefer the 5204R over the P or 1R, but doesn't mean that others are any better or worse..
Regarding depreciation, since mechanical watches are supposed to last generations, I don't believe they would depreciate like a car would. I always assume a new watch will take a hit as soon as you swipe your credit card and they write your name on the warranty card, unless of course it is a piece currently enjoying a lot of hype. Generally speaking Patek holds value much better than other brands.
As for the warranty card, I have no clue to be honest! Hopefully others can chime in.