I would, except I sold it already lol... Just kidding actually. The few coins I've ever sold were "extras" anyway. Well OK, I'll admit that financial conditions forced me to sell my 1976 gold sovereign a few years ago. Would post a pic but I don't have one. Hoping to buy one back some day. That's really the only regretful sale I can think of though.
Machins Mills copy of a British half penny with the date 1776. The British didn't mint any half pennies in 1776, because they were too busy fighting some uppity colonists.
I have sold many that I really enjoyed having in my cases, but two that I really miss. Both were sold at the fun show to other dealers. The Walker looked even better in hand, the most beautiful walker I have ever owned. This is the best looking 1904 Peso I have ever seen, hands down. I hated to see it go.
I don't regret selling any coins I've owned, but I do regret trading a Jerry Rice RC for a bunch of mid-90's basketball card packs at the flea market. I bought it for a nickel. Four sharp corners. Ugh.
Here is a coin I sold that I wish I had kept. It doesn't keep me awake or anything, but I do remember this coin well. Something about the toning that just stands out to me.
I wish I hadn't sold this one: Color corrected: I don't have very good photographic records of it either.
There's nothing I've ever sold that I wish I still had - however there are several Morgan CC's that I wish I had sold for more money