The three I've sold and regretted I've bought back again.

Sold Steyr LG110 in favour of a Daystate Grand Prix. Eventually went back to the Steyr although I did like the GP.
Sold Walther LP400 in favour or Steyr Evo E. Have just bought another LP400 last week. Have both now. (One will have to go...)
My first air rifle, a BSA Mercury Challenger (aged 14). Sold or traded that in for an Air Arms Firepower (still around 14). I managed to aquire a boxed like new BSA Mercury Challenger a few years back and more recently an AA Firepower too.