It's easier to blame someone else, seems to me like Adam is just the fall guy in this scenario which is unfortunate.
Who was supplying Adam the gearboxes in the first place, are the UK based? Why not go and see them in person? Be proactive, don't just sit behind your computer screens moaning that someone, who can't pay you back owes you money.
Never have I been a member of a forum that has so many members out to fuck other people over, and I've been using them for about 13 years now. Perhaps it's the age of driver the Starlet attracts? Who knows, but I can't trust anyone on here at the moment, PayPals protection should keep you safe, although I'm not convinced there isn't a scam out there that will have them siding with the seller should the buyers goods never arrive.
Protect yourself as best you can, or buy new from reputable suppliers.