Nice of the seller to have the original engine and let you know that it doesn't go with the car. The buyer will have first chance at buying it but for how much?
I get not including the extra options and aftermarket new parts he bought separately from the car but not including the original engine and interior is just wrong. It will make it more appealing to the LS swap crowd. Such a nice original highly optioned car like that should be kept whole.
There were 949 69 birds built with a power bucket. As far as I know they didn't keep records on the number of 68 and 69 leather seats that were installed but based on the known and verified cars that have been found my guess would be that leather is far rarer.