first, it is not that rare, second, the spedo sensor is very easy to break if you dont know what you are doing when you might have to pull the tranny, being a "mechanic" he should have put in more info as fair as oil changes and regular care if it was going to be his "keeper", and he says "it runs and drives!" he did not say it runs and drives great or good or anything, that tells me that he is saying, yeah it runs and drives until you get it out of my driveway and he dont care,. go and meet him and take it for a drive, make sure the spedo works, if it does then check to see if the sensor looks new, they are the same for the mph gauge working as the miles turning. what i mean is that if it really has that low of miles then the rear main should not have gone out unless there is some thing else wrong with the trans shaft or the crank shaft, like something to do with the bearings,.
even still, like said before, 1500, it is still a 20 year old car.
good luck.