As the PP said, many of the items will the be same at all four candy shops.
Tigger Tails are unique to the Candy Shop at Pooh's Corner. They have similar variants (Cheshire Tails, Mickey/Minnie Wands, Olaf Wands) at the other candy shops. These are basically three jet puff marshmallows, dipped in caramel and then milk, dark, and/or white chocolate and decorated to resemble some character.
In general, you can find the caramel apples, cake pops, and rice crispie treats at all shops, though the specific designs might vary.
In terms of the hand-made treats, it seem like some of the ones made in the Candy Palace show window are only sold in the Candy Palace. I haven't seen the Nuttles over at Trolley Treats or Marceline's (the pre-packaged ones are variable at various locations, but the hand-made ones seemed more limited). I can't remember about the peanut brittle (English Toffee was available everywhere it seemed like).
Trolley Treats, when it first opened, specialized in hand-made marshmallows. These came in different flavors and with toppings, but they have been discontinued. They were lighter than regular marshmallows.