If the Jeep is being sold as a running and driving vehicle, tell the buyer that you are coming in from an hour or so away and see if they will meet you half way. The goal here is to see if there are any leaks that have been wiped up. The specific seal you are looking to see if it is dripping is the rear main. On the 4.0L, the rear-main is two-piece seal. You can actually swap it out without removing the transmission. This is another DIY project for those wrench-savvy enthusiasts. Expect to pay a few hundred bucks if you have to farm the job out.