Contrary to many modern pastor's advice, I don't think a pastor should purchase someone else's sermons in order to preach them. I don't think they should NEVER do it, but certainly no more than once or twice a year or one sermon series a year. I am talking about pretty-near complete sermons or outlines. I think a pastor should read and listen to sermons on a regular basis.
No idea is completely new. Good sermons inspire and inform your preaching. Like reading is to the writer, so sermons are to the preacher. (I also think preachers should listen to good comedians.) Listening to good spoken-word performers helps the preacher learn timing, audience interaction, pacing, and flow. Listening and reading other people's sermons give you new ideas and fresh approaches to topics.
There are several reasons you should not preach other people's sermons:
1. Integrity. Whether we like it or not, the people in your church expect you to prepare and preach the sermons. We don't approach this from a legalistic, bow-to-their-whims approach, but simply recognize the expectations. The integrity issue comes from taking credit for the arrangement and presentation of ideas that are not yours. In order to maintain integrity, you should be honest with where you got the sermon. So the question becomes, Are you willing to tell your people every other week that you are preaching a purchased sermon?
2. You are not "Fill in the Blank." You are not Bill Hybels, Rick Warren, Ed Young, Rob Bell, or any other star preacher for that matter. Unless you buy every sermon from a particular person, mixing and matching sermons from several different authors and yourself is like taking one chapter from Stephen King, one from John Grisham, and one from Danielle Steele and passing them off as the same author. It just doesn't work.
3. The star preachers don't know your people. They don't write their sermons with your people in mind. You know the needs, hurts, sorrows, joys, and passions of your people; therefore, you should be the one deeply praying for them and crafting your sermons to address the needs of the community.
4. You are the one who is called. This is probably the most important aspect of this whole post. When you preach other people's sermons, you shortchange the calling of God on your life. God calls, transforms, and uses YOU to prepare a sermon. God wants to speak through you. Granted you may not be as good as one of the star preachers, but God didn't put that star preacher in your pulpit; He put you. God wants to use your voice and your mannerisms and words to speak to these people.
If you preach someone elses sermons on a regular basis, you have no opportunity to find your own voice. Finding your own voice takes time. You have to read and listen to sermons. You have to learn the basics. You have to practice, practice, practice. One day, you will a voice that is distinctly you.
When I was learning to play the guitar, I read an interview with the great Santana. He said (paraphrasing) take your lessons, practice, learn to play the classics and stuff you like, but one day lock yourself in a room, turn off the lights, and play until you sound like yourself.
This thought from the great Santana reminds us that we are never separated from our influences, but, through us, our influences come out in a unique and different way.