Worth $600 used? I know they are old and don't have all the features newer housing have, but seem robust.

Probably. They're a great housing and I still have a pair of them. I haven't paid a ton of attention to night vision stuff lately, but I'm guessing a used RNVG housing is probably around that price, or slightly more.

Edit: I just saw your other post about picking up a pair of Anvis 9's. If you plan to swap the tubes and optics over to this Sentinel housing make sure that it is thread for anvis optics and not PVS-14.