I bought a mint G version from KEH for $450 ish last year. It was mint, box and all. It wasn't a deal, just a fair value. I also bought a (new) Sigma 105 OS) macro F2.8 for $7xx from B&H. I think you should look at the real market before you throw out a price.
The lens market is not like the body market, as you still have options for new.
This is the F1.4 version. The f1.4G version sells for over close to $1600 new. Either KEH made a mistake or more likly you've made a mistake. If you got a 1.4G lens for $450 from KEH then that is the deal of the century