How to check in a Rules Condition if an Ubercart product is out of order?
As per the UC issue about "Stock transaction should be more flexible through Rules" (from 6 years ago, status "needs work" ...), it seems that at this time trying to use the Rules module for such "out of order" situations may not be a good idea.

You may want to try to use the Ubercart Out of stock Notification module instead. Some details about it (from its project page):


  Out of stock notification for
  Ubercart.
  
  It uses ajax and jquery and asks the server for available stock and
  replaces the 'Add to cart' button with some configurable HTML.
  
  ...
  
  It also provides server-side validation so this module can prevent the
  addition of out of stock items even if JS is disabled or if the user
  clicks on an "Add to cart" button too quickly.