The Twist: The entire film is a setup. Juan is not a rookie; he is a master con artist and the boyfriend of Marcos's sister, Valeria. The 'Nine Queens' stamp deal was a fabrication designed specifically to entrap Marcos.
The Plan: Every obstacle Marcos faced—the thieves on the motorcycle, the stamp expert demanding a bribe, the Spanish buyer's demands—was orchestrated by Juan's team. The goal was to get Marcos to use his own $200,000 (which he had hidden away) to buy the rights to the stamps from the 'widow' (another accomplice).
The Outcome: Marcos hands over his $200,000 for the stamps. He sells them to the Spaniard for a check. Juan and Valeria's team splits Marcos's cash. Marcos goes to the bank to cash the check, but the bank crashes (shut down due to fraud), rendering the check worthless. Marcos is left penniless and alone, while Juan, Valeria, and their team walk away with the cash, having exacted revenge for Marcos stealing the family inheritance.