My own stubborn character and the belief that there had to be one final 'secret' trade upgrade for an Accessory in Rune Factory 2 created considerable labour for me as I made Accessory after Accessory at my Workbench and upgraded each to Level 10 before trading it to discover whether or not it would be transformed. I rather suspected that the Accessory I needed was the CHAMPION BELT but that was one of the Accessories with the highest Skill Requirements in the game, so I doggedly continued to upgrade other Accessories to Level 10, knowing that each upgrade would increase my own Skill Levels even if it proved ineffectual in terms of a trade transformation.
It always is rather satisfying when a guess is proven correct. The moment when I saw the prompt to the effect that my item had been upgraded, I felt all the work I had done was worth the effort. A more sane player probably would have requested the name of the last Accessory from the makers of the game... but I'm glad I did it my way.
Here is a brief video that shows the complete list of Accessories that can be obtained as trade upgrades as well as one of the very best Accessories in the game.