Never officially sold to the public, but a handful were purchased from Geissele. The $5k GunBroker listing above is from a few years ago when there weren't any of these triggers in private hands. Now you can grab them from collectors who have them for about $2,000 or a little less. There's probably 2 or 3 dozen of the real Mk13 Geissele triggers currently in private collections, so they're still pretty rare.
Hopefully it's one of the new-old-stock triggers that came directly from Geissele, and not one of the broken and unsafe triggers that came from DRMO. If it's a new-old-stock trigger from Geissele, then $1,600 is actually a really good deal. However, if it's a DRMO trigger, there's a reason that it was DRMO'd, and I wouldn't even pay $100 for it.