Translation:I don't know when I bought this watch.
No, while they both translate to "when" they are not the same. "Πότε" asks a question about the time something occurs: "Ποτε το εμαθες;" When did you learn it? "Πότε θα πας;" When will you go?
όταν is a conjunction and tells us when something happened or will happen: "Όταν αυτός έμαθε τα νέα .. ". "When he heard the news..." "Θα έρθω όταν τελειώσω τη δουλειά μου." "I'll come when I finish my work."
Btw be careful with "ποτέ with the accent on the "ε"...that's another word entirely and it means "never".