I just did a test with the event tickets and got all the pieces for the Stonewisp Truth and Law pet.
While I was holding the fragments, their respective pieces in the vendor are marked 'Collected' and highlighted in red. Once I combined them, those same pieces are now flagged as 'not collected' and are no longer highlighted in red.
This means, unless you have a mod to track collectables, you run the very real chance of "double buying" fragments, especially if they show up on the other vendor that sells past event fragments.
Seeing as you can buy tickets for $2.50 (250 crowns; $0.01/crown). This is a glaring issue.
You have to learn the rules of the game. And then you have to play better than anyone else.