I use an Excel spreadsheet that someone posted over at Glocktalk. Closely approximates the format for the FFL bound book. Come to think of it. I've got to back that damn thing up. I've had to reformat the hard disk on this computer 3 times.
My big mistakes in selling were:
629 Mountain Revolver (one of the first run 1000)
Beretta 85F (man I loved that one - convinced myself I didn't need a .380
)
S&W 6904 (totally reliable and very accurate)
Taurus PT99 (the first handgun I bought after turning 21) - I don't miss it too much, I replaced if with a PT99 5 years ago.
AMT Automag II (very impressive firethrower).
S&W M19 2.5" (this one hurt bad).
Not so big mistakes:
Ruger Super Single-Six (couldn't hit the broad side of a barn with it)
Golan .40 S&W (interesting gun, but also about the same size as a Sig P226)
CZ100 (never could get used to the trigger, liked just about everything else).
Marlin .22WMR bolt-action (didn't really grab me).
Ruger Vaquero .44 mag (accurate only with fullpower pedal to metal loads and abominable fit and finish).
.
Even now in making this list, I'm amazed at what I've had.