This is being sold without the STC paperwork from S-Tec.
This is the area of true concern so buyer beware and don't blindly jump into the water. S-TEC is now owned by Genesys and they have their own attitude and outlook on life.
Any potential buyer should have a one on one discussion with Genesys on the front side......don't buy this and then call them thinking life is good and this is an easy process.
I think you'll find that Genesys will not only charge you for the STC but they will also require that you send them the unit so they can certify it for your plane and your plane only.
Just a heads up.
Now, past that......they do make rock solid units. I own 3 and love them. I just wish they (Genesys) weren't so, well, uppity....... but that's life in aviation these days.