Used guns tend to go for about 90% MSRP downwards with use and wear. IPSC guns tend to be WELL used, or used barely at all. If it's well used, and runs well, It's probably a good buy as a starter gun if it's well priced. If it's somebodys FrankenSTI, price it for the value of the frame and mags only.
If it's LNIB, odds are it was an impulse buy that didn't feel right to the origional owner, or they have switched to open or production.