I'm considering buying a 92 Ford E150. Haven't gone to see it yet but the ad says it won't go over 15mph. Could this be something simple, or is a tranny in order? 237k miles and it's got a 5.0.

 

There could be several problems that's causing the van not to go over 15 mph.

If it is the transmission, even stuck in 1st gear, it would rev up and go over 15, just not up-shift to 2nd gear.

Keep in mind, that it might cost $1,500 - $2,400 for a re-man transmission, that's off the shelf price, to an installed price.
Also check out if this is an AOD or E4OD transmission.
'92, 5.0 should be a E40D, but it was a change year, and could be an leftover fitted in there.

With the mileage you posted, it could be time for the transmission to be changed, and it also could be time for a new engine. Or both??

Might be worth to look at if the body and suspension are in good shape, and the price is right to replace the complete drive train.

Other wise, it might be a new candidate for the junk yard?

 

Think about it- if it was honestly something simple, why wouldn't the seller fix it and sell it for a lot more? My guess is they've had it checked out and know it's nothing simple (re:cheap) so go in assuming you have an expensive repair and not some super lucky great deal. Set the bar low so you don't get disappointed if it is the worst case scenario.