In 1914 the Greek revolutionary authority of Chimarra issued this set of 4 values in a very small quantity. At that time postal service of that area was limited and only few copies are saved to nowdays. Mailed copies on covers are extremely rare. Thus catalogue value of this set is over $1500.
 Today ebay is full of auctions offering this rare set mint, used or on covers.  Unfortunately most of them are sold to victim-collectors that think that they bought a rare set for only $50 or $100. ALL are forgeries!!!. Known forgeries with a real value of less than $10.
 I saw at list 100 sets & covers bearing this set sold at prices more than $50. Poor collectors beware!!!
 Now some tips:
 The genuine copies are very easy distinguished by the paper used. You have never seen this kind of paper at another stamp.
 The printing is poor & oily. You may see from the back the front icon.
 All forged covers are address to Mr. Gogakis ( in Greek or English) either in Corfu or Athens. ( Backstamped forged properly).
 Most used sets on pieces are cut offs  from these covers. There are hundreds of them.
 My advise is when you buy forgery pay for a forgery.
 For the history matters, unfortunately the revolution of the Greeks in Epirus failed twice, due to fascist Italy that supported the creation of a new vassal state to its interest, Albania.