Trazodone for sleep while on Buspar?

There is a slight risk of an interaction that might cause a condition known as serotonin syndrome. It's rare and easily treated in the early stages. Your doctor most likely has determined that this risk is outweighed by the benefit to you of the combination.
You should acquaint yourself with the symptoms but I wouldn't be overly concerned. This link will explain it and if you have more questions you can talk to your doctor about them.
https://www.drugs.com/cg/serotonin-syndrome.html