If the seller had sold the book, after being explicitly told by the original buyer that they were not going to pay for it, then no that buyer would have zero grounds to nominate the seller to the PL if they then sold the book to someone else.
Even if there are private communications dated before this public PL nomination indicating the original buyer who reneged was willing after the fact to pay in a "couple of weeks", those weren't the original terms of the sale and the seller is under no obligation to entertain much less accept those new terms.
Regarding increasing instances of non payers and it not being reported/made public via the PL nomination forum, if we are going to have zero tolerance then how do we encourage members to not shrug off those bad deals and go through the hassle of nominating the dead beats? It's either a big deal that the sales forum, and the boards in general, are a safe place to participate in the hobby for both sellers and buyers, or it isn't. No?
I'd rather we see more nominations, and those conflicts get resolved and any dead beats becoming known quantities to the rest of the community so we are aware, then the boards be filled with non payer landmines because too few members feel like dealing with the hassle.