If “not shifting into gear” simply means you can’t move the shift selector, then the problem might be as simple as removing and replacing the ignition/brake/shifter interlock in the steering column.
If so, that is not a difficult problem to fix and, in fact, can be quickly overridden by opening up the top ofthe shift console and removing the interlock cable from The side of the shifter mechanism.
That said, the current owner might not want you taking apart the car in his driveway and if you are unable to test s drive the car, you will not be able to determine what else might be wrong with it.
However, if you can ALSO tell from Carfax, other maintenance records and overall condition that the )eep is in otherwise decent shape , it MIGHT be a good deal with minimal risk to still buy it IF the shifting isssue is in fact only the interlock mechanism.
Just my .02