This was not your first time shopping in the magic shop. In fact, your pevious trip is why you've spent the last few months as a man instead of a woman. But although you're still enjoying being a man, the novelty has worn off. You want a more exotic transformation.
You eventually come across the transformation watches. Out of all of them, the Role Swap Watch grabs your curiosity the most. Whatever it is, it sounds more interesting than just a basic transformation. You look over the instructions. Swapping roles with another human sounds boring; you're looking for an actual transformation. Although, swapping roles with an animal sounds like a fun twist.
But then you read over the 3 o'clock position. "Swap with inanimate objects or body parts." You hadn't even considered being a body part. If you wanted an exotic transformation, that'd be it! Granted, you'd still be a human man. But the twist is too interesting to pass up. The question is, how would it work? The instruction gives an example with an object, but not a body part.
Bringing it to the shop owner, you ask about it. He doesn't know how it works, either. Owning a magic shop grants him immunity to all the shop's magic. The upside is that he's immortal. (Mental note, try to become immortal.)
Having already learned firsthand that the magical items really are magical, you decide to just take a chance with the watch. At the least, the old man is willing to offer a refund if the watch's effects aren't as desired.