There are no coupons.
Based on past experience, Obihai's sales often correspond to major US holidays, so you'd probably have to wait for Memorial Day.
Don't buy a used OBi product, especially on Craigslist or eBay. It's highly likely that the device is defective/fried. It's worth it to buy it new, from an authorized retailer: Amazon or Newegg or Fry's, and get a new unit with a warranty.