I find myself addicted to going to gun auctions...one every two or three months. I buy both handguns and long guns, with the attitude that for every three I buy I will keep one and sell two to pay for the one I keep.
So far during the past year that has worked well for me, without doing a lot of damage to my retirement income (and losing my wife's cooperation).
Shotguns, even higher priced shotguns, seem to go 'cheaper' at the auctions and yet on the on-line web-sales sites they earn good return. Handguns pricing at auctions really varies a lot from event to event...but the hardest group of all are the Center-fire Rifles. The tend to go high at auction, especially those with the "cool" look (AR-AK etc) which makes reselling a low profit exercise.
You are correct about the satisfaction you get from using your retirement time and money for something you really enjoy. !! Keep it going !