While helping a friend buy a 2003 BMW 325i (E46 model) we found a problem on a car he wanted to buy. The serpentine belt had just been replaced by the dealer 500 miles before and was already showing signs of fraying.
The seller had documentation showing it had been done and we didn’t have any reason to think he was not telling the truth. When still, the belt looked brand new. But there was obviously a problem with the belt that could be seen with a flashlight while the engine was running.
Digging a little deeper, I called a local BMW shop to get a replacement quote and was told that “the pulleys on those models tend to fail and that would shorten belt life”. Then we found this video from BMP which further explained the problem:
So in the end my friend bought an even nicer E46 325i from another seller. Sure enough, the belts and pulleys had been replaced under warranty.