Don,
Unless the seller has a really good reputation, buyers will assume that stamps offered on
ebay at auction are defective and will bid accordingly. It appears that the price for a decent used copy would be in the $40 range for fine centering, in line with the Scott Valuing guide. A few at auction have sold quite cheaply, but perhaps the stamps are 136 instead of 136A. If the seller cannot determine the difference between an early or late state grill or fail to provide a back scan, buyers should be wary.
Among the listings currently running is a badly rendered image of a pair with fake grills. No wonder buyers bid low.
Clark