I once parted with a Colt 1911 pistol that I was pretty fond of and used the money to fund some other pistols. I later regretted it and bought it back, and then found that it wasn't nearly as much gun as I remembered it being after using a few others since. I sold it again a couple of years ago and never looked back. No regrets this time.
In contrast, I bought a CQC-7 in 2012. Then later the same year I bought a CQC-8 also. The 8 got a bit more pocket time and I needed cash so I sold the CQC-7 to a good friend. Later on I found myself looking at getting a brand new one, but something inside me told me to get the other one back instead. I contacted him and asked what he would take in trade for it. I ended up giving him more in the deal than what the CQC-7 was monetarily worth: A Brous Silent Soldier with the newer style and a BRKT Bravo Necker II in 3V. Even though those were great knives and worth a good bit more, I have not regretted the trade once. I have re-bonded with the CQC-7 very strongly and I don't think I'll ever let it go. It's actually one of my favorite knives of all time - And having back my first Emerson has made me a very happy man overall. Something about the CQC-7 design just clicks with me. It was a knife I always wanted when I was younger and in my earlier phase of knife obsession. It was out of reach but for some reason I always wanted it and then finally got it and kept moving regardless. Now I appreciate it more, and it's sort of become representative of who I am as a knife lover.
Another knife I couldn't part with is my SnG. My parents took a great liking to my wife, and for a few years we've been living with them and they've been treating her like the daughter they never had. She gets spoiled a good bit by my parents, kind of a sweet thing I think. One day my dad asked me if I could have any knife regardless of price, what would it be? I told him a Strider SnG and showed him a picture. He asked what it cost, and I told him and he sort of freaked out. Then he said "Start saving for it, and I'll give you $300 toward it so you can afford it." He had felt bad about spoiling my wife so much and not having done anything nice for me in a while, and I took him up on the offer. I ended up getting the Strider with $175 out of my own pocket thanks to his contribution. Can't let that one go.
EDIT: Last couple I forgot to mention. There's an old Schrade I found in my grandma's house when I was a teenager. We were clearing things out after she had passed away. I've always held onto it and it won't go anywhere. Also, I still have my very first pocket knife that was a China or Taiwan made knife under the Beretta name bought for me before going on my first camping trip with my dad. My parents always held onto it with other camping gear between trips so I never had the chance to lose it. Now it's in my possession and I could never get rid of it either.
I've always had a love for knives, and I'm sure there will be plenty more I couldn't part with. Right now I have a whole drawer full of knives I don't ever PLAN on selling because they were gifts and hold some level of sentimental value, but those four specifically mentioned would stay even if I absolutely, positively needed money even in an emergency.