I looked but couldn't find the part where he "try's to work it out with the seller". In this deal both parties stepped in it, and as such both sides should look for a solution where they both have a soft spot to land because that's how FABO members generally deal with each other. Post #69 among others tells me that he talked in circles about wanting to reach a reasonable compromise, but in the end just ran and hid behind Mommy Paypal's apron. Maybe Riddler has another part that he needs that Riddler could give him a smokin deal on to bring him his badly needed justice. Maybe he could have sold the incorrect brackets for a lot more than he bought them for, which was offered. Lots of ways to work this out if parties actually have an interest in working it out.
As for "6 pages of lynch mob", sorry don't see that. I see calls for compromise and distribution of responsibility, and lots of anecdotes from "the real world". Treat people the way you want to be treated. That's not complicated.
Piling on the return shipping is a dick move - even you acknowledge it was most likely out of spite. Take that crap to a different forum. Don't be a stain on FABO.
Let the buyer limit his future purchases of used parts to the retail used part industry (see my earlier post above) where his "win at all costs" attitude will dovetail nicely with their "take it or leave it (less 20%)" business philosophy. Him, them and Paypal all deserve each other, and he can look to them for support, guidance and answers for all things A Body while paying for shipping both ways.
