Hello, the idea behind the legendary-specific rerolls was to give some love to Pokémon that usually never get rerolled. Using a normal reroll on them would feel like a disadvantage or a waste, since there are Pokémon that are far more relevant in PvP or PvE than something like Celebi.
If we made these tickets tradable, sellable, exchangeable or anything similar, it would completely lose their purpose and make the legendary-specific reroll tickets pointless.