I am looking for alarms to purchase, and I just wanted to clarify something:
Do TrueAlert horn/strobes need to be connected to a Simplex panel or a SmartSync module to work?
No. Those are only certain models. Almost all that you find on eBay are either SmartSync or Addressable. There is a model number checker on the main The Fire Panel website you can use to check if a device is free-run or one of the other models.
http://www.thefirepanel.com/fabg/simpcheck.php