I usually don't sell cars, so my biggest selling regrets have been

70's Fender Stratocaster, cream with maple neck, Schallers
70's Les Paul Custom, Dimarzio PAF and Super Distortion
60's Telecaster (removed the Fender "bigsby")
70's Jazz Bass
Ovation Custom Balladeer 12 String w/Barcus Berry
Fender Twin Reverb with 21" Altec Lansings