It could be a ton of stuff wrong or little or nothing. Could be a fan, could be a fuse, could be that it is 15 years old and been mistreated. Read this:
In any case, this IS a 15 year old WJ. This model jeep is known to be plagued with problems, especially with the heating/AC system. If there is really no reason you HAVE to have an old SUV, then I would walk away.
If I HAD to have this one, I would tell the owner that I would consider buying it AFTER he got the heater/AC working to your satisfaction. Not before.