I think amicably if the seller send a 120mm standard fan, and a photo copy of the courier invoice charges, and the the buyer reimburses 50% of shipping -- we are fine. No one has to own up to anything. Mistakes can happen, and a critical insight was over looked.
Gowtham, would really appreciate your cooperation here. Also from next time when you send 2nd hand goods, please mention explicitly if some parts/accessories/assortments which are part of the standard bundle, which you have put up for the sale --- are missing. This way your intentions are clear from day 1, and doubts are not raised on your integrity.
Saz, you can accept the 120mm fan and reimburse the 50% shipping charges ELSE you may ship back the whole package at your own cost and expect the sales refund amount from Gowtham.
Hope something can be chosen here.