Sounds like a good deal
It looks to be just what he said. When a dealer has to replace a blown motor in a Corvette, they get a long block from GM and just transfer over all of the accessories, coil packs and yada yada yada. The engine looks to be just a GM dealer long block.
Maybe an email to the seller might give you an idea of how he got it. I see a locker room in the back ground of the picture and I don't think it's Golds Gym lol. Maybe it's something being sold from a local dealer?
I am not 100% familiar with the LS1. Does the LS1 have that long tube coming out of the water pump on the top? The water pump on my LS3 looks different.