(Dratini nooooooo!- Image © Nintendo)
First of all, skip eBay when purchasing your new PokéWalker, as they run anywhere from $25-$30, often without shipping. You can actually buy a brand new one from the Nintendo website for under $15 with standard shipping.
Additionally, you don't need to restart your game if you should wind up with a PokéMon "stuck" in your Walker. Via Bulbapedia:
"A Pokémon that has been transferred onto a Pokéwalker isn't actually sent to the device, but rather copied and the data for the Pokémon is sent. The targeted Pokémon is instead put aside, and can't be used in the game, until the data has been returned to the game. Therefore, if a Pokéwalker is lost or damaged, the Pokémon is not lost. Pokémon can be restored to the game it was taken from if a Pokéwalker is lost or broken by pressing and holding Up, Select, and R at the Pokéwalker connection screen. The Pokémon will be returned to the PC box with its level or happiness unaffected."
So all is not lost if you lose/damage your PokéWalker, although ultimately, I'd still advise you just keep an eye on it, rather than having to purchase a new one.
(Image © Nintendo)
NOTE: I got my new PokéWalker, and had to look up how to reset it. I found my solution, and the new Walker is working wonderfully, but so you know, while you will be able to keep all the paths you've unlocked, you will also lose all of your watts. Bollocks.