Do I regret selling certain cards? Yes, of course, mostly because some I’ve sold for ‘spare change’ are worth ten of thousands of USD nowadays.
Do I miss any cards that I’ve sold? Nope, not really.
I barely sell to begin with, and if I sell, it’s in 95% of the cases a duplicate. The only few non-duplicated cards I’ve sold were either to:
- Help a fellow collector (usually species collectors, but sometimes an illustrator collector)
- Buy a house
And neither of those I’d ever miss enough when I see what I got back for it. 
Greetz,
Quuador