From what I have read, it's most likely a re-arsenaled 16" carbine with detachable AK mags. Are these reliable? I know the ones with the duck billed detachable mags are crap, but don't know much about the one's that take AK mags. They are asking $650 obo, which seems a little high, but not sure on this specific model.

Probably better than a duck billed setup but the difference is the gun you are looking at would usually have a stock sks mag release unlike the better  release on a m, d, or sporter model. 650 is kinda high around here and im sure if you look around a bit a m or sporter would find you. One thing about those ak mag setups is that a pmag or such may require a bit of fitting but will hold tight in the mag well. Hope this helps.