I looked at a 92 f250 7.3 idi n/a auto. Guy says the truck lost forward speeds and then all of a sudden locked the motor up. I thought there was no way to lock the motor up with a torque converter since all it is is fluid moving over vanes, but can this actually occur? Truck has plenty of oil and coolant in it, and no metal shavings in the oil that I could see. Also, is a truck with a blown trans and engine with no solid axle front even worth $800? Body has usual rust on the bed and interior is dirty, but all there.

i've heard of TCs blowing themselves to pieces, which could then have a chunk locking it up against the bellhousing, or if the end where it goes into the transmission pump were to get messed up, it could lock up in that point.
i certainly haven't heard of it happening.
i would suggest you put a breaker bar on the front pulley of the engine and see if you can get it moving either direction. that should help you get a feel for what happened in there
by the sound of it, i'm guessing the problem is in the flex plate or TC

Do autos have an inspection cover like manuals do where I could possibly see inside of the bellhousing?

yes.down below.but navistar pulled a real sweet trick.
they placed one of the inspection plate cover bolts in back of the starter! lol (not that they had much of a choice though.)
so you've got to pull that out first.that's always fun in itself.tip; leave that one cover bolt out.
imho,it's not priced correctly to be worth your time.it could be a locked up engine due to neglect of running proper diesel coolant,which is more likely.
there's no D60,there's no turbo,it's an f250 so its a srw there's no dually axle.not a whole lot left to a stock f250 pickup with the auto trans and engine both in question,besides what she can bring in scrap,should nothing be any good......unless you've already got an engine/trans combo ready to drop in,then it could be worth $800 to ya all easy if the frame and body is fair enough.