Working out a deal with a buyer on Facebook Marketplace to sell three City volcano sets to a mother for her son that were combined Christmas/birthday gifts.  I did not make as much money as I wanted but hearing her say thank you and the look of joy on her face was worth so much more than what I made.

Selling two Chima sets (yes, I said Chima sets) on Facebook Marketplace for a slight loss that I bought for my kids that they never opened.  Sometimes it is better just to unload sets so as not have to look at them anymore.   

And as I have mentioned before, messages like the one I got on November 29 on BrickLink after working with a customer on the sale of a 41179 Queen's Dragon Rescue set:

"Thank you so much for your quick responses and smooth transaction. This is my first order through BrickLink. My daughter will be so thrilled to get this dragon under the Christmas tree this year! She has been asking for it for a very long time. I was crushed when I discovered it was retired. But I am so thankful to have found BrickLink. Thank you again! Have a wonderful holiday season!

And thank you for the shipping refund. That was very kind of you!"
 

QFLL mid-late December 41597 Go Brick Me, sold out at lego and pretty much everywhere else at rrp, Tesco 50% instore £13 (with £26 shelf-tag ), easy ebay sales £38-£40