Writing my own book! Adventure/Time Travel

I'm attempting to write a book that is kinda an adventure book. My idea is that a woman finds an old watch while cleaning her apartment, - (Still figuring out a location for the story to take place at) she takes the watch to a antique clock shop to sell it for some cash. - She knows how much it's worth and gets annoyed at the manager for his lowball offer to buy it from her. He offers so low because of how bad his business is. Most people come in just to browse and not buy, he needs money but can't offer any. They both try to take the watch when they suddenly go back to the day BEFORE it was sold to the public. She needs to buy the watch and go back to her time, but in the process will leave the owner of the clock shop behind. In order for the owner to get back to the present he will have to kill her, if he does she will well die leaving a big mystery before their time, how she has a phone, her clothes and why it says she doesn't exist/no one knows her.

Only ideas and I'm open to yours! The time they go back to, and place it takes place in. Any other ideas for the story! If I do finish it I probably won't get it fully published. If I do wanna finish it it will take a while and I'm still in school. 

Thank you! Alu.