I wouldn't condemn the trans that's presently in the car just yet. If the fluid level is super low, it's not gonna work. Ditto the filter if it's clogged or collapsed (as you mentioned).
I'd drain the pan and the torque converter. Replace the filter and fluid (with the proper amount) and see what happens.
Did the seller have any info about the transmission? When/why/how it stopped working?
Did he try to get it goin or just park it?