He is now demanding his $75 back or will give me a bad review.
and this is where he will come unstuck. The feedback extortion policy states that "Threatening to leave negative or neutral feedback for another member unless the other member provides goods or services not included in the original listing is not permitted". Your returns policy for the fridge was clearly stated as "The seller won't accept returns for this item". He wants you to provide a service that was not included in the original listing.
If the buyer is threatening to leave a bad review if you do not accept return or provide a refund then that is where he has erred. You do not have to give a refund, but even if you agreed to a refund he would need to return the fridge to you.
just as an aside, your comment that "I have a 100% rating because anything I sell, I sell ethically, knowing that the 2nd hand item is great value and in great working condition" is not actually true. You have 100% feedback simply because you have no negs in the last 12 months. Feedback has nothing to do with ethics or selling great value items but everything to do with buyer perception of a transaction. There are some buyers that are totally unreasonable and dish out negatives to the best of sellers . . . . . and this does not mean those sellers are not ethical or selling great value items. Your 100% feedback is based on 57 feedbacks received in the last 12 months . . . only 3 of which were as a seller, and your 54 feedback as a buyer MUST be positive because a buying transaction can only get you a positive feedback. A more accurate statement about your feedback would be that you have 100% as you are primarily a buyer and have been fortunate enough with your three sales to not have struck an unreasonable buyer. If your current unreasonable buyer does leave a negative that reduces your feedback below 100% it will not mean your are unethical or don't sell great value items.
