Most online stores do appear to offer warranties for used or rebuilt motors, though the length of the warranty varies depending on the manufacturer and the motor's horsepower. Sites such as iboats.com that sell used or refurbished motors show the warranty length for each type of motor.
Classified sites where independent owners offer used motors for sale or trade, such as at Bass Fishing Network, are a different matter. Most of the posts on such a site don't mention warranties in their main entries. However, even motors purchased from independent owners sometimes have warranties because warranties are transferable, notes Yamaha.