help! Engine seized on toyota

I bought a used car about 20 days ago that was supposed to have a rebuilt engine/transmission....and it actually purred like a kitten...no smoke coming from oil area/no pressure at all...absolutely no noise.
it has been about 750 miles of hwy miles since purchase, and all of a sudden it locked up on the hwy. Dead.
There was transmission fluid on the ground before towing and at repair shop right after.
Transmission was serviced less than 8 days ago by the seller, who also was the one who "replaced" the engine/transmission with one from salvage/wrecked car....
Is it possible that in that short period of time, with no existing problems/leaks that were evident, that a car this old could have used all of it's oil reserve and run out if it hadn't been checked since purchase date 2 1/2 weeks ago?
It appears to have no oil in it, but I don't know if the repair shop drained it before/at diagnosis time....?

oops...i didn't mean to put this in the "off topic" section....sorry.