Don't have regrets. When we sold our '69 SS AMX for $45K and 2 years later it sold on E-bay for $130K, I had no regrets. We made $20K and there was lot of stuff that was incorrect on the car that would cost $20K to fix (the motor wasn't correct for a '69 SS - later model heads, block, carbs, etc.).
The guy who paid $130K basically got screwed, so let the buyers beware out there.
Funny story about the GN. The 72 YO buyer shows up Saturday morning to pick up the car. His 70 year old wife with mild parkinsons is going to drive the car back 8 hours to Pecos. I told him the 27 year old tires won't make it in 95 degree heat @ 75 - 80 mph (the speed limit out there). No problem he says, he'll go to discount tire and "get rid" of the old tires. I reminded him those are the original correct tires for the car and being a 30 year member of the AACA, he realized that would be a mistake.
Under a layer of junk in my back garage I found him 4 pretty nice GN wheels for an extra $200 and told him to ask for 235/60's at Discount tire (they had $80/tire Coopers) and he was off with a big smile.
He also just bought a '58 Buick 2dr with 45K miles a couple months ago, so he's building a nice collection of low mile buicks.