Seller says the bumper has drooped ?
They don't droop of their own accord.
The bumper beam is mounted to two bumper irons bolted to the car's structure.
This is one of the irons :
( The steel tube sticking out under the headlight with another tube welded perpendicular at the end ).
There's no scope for it to droop naturally.
The perpendicular tube does have a method of height adjustment for the bumper by means of a screw adjuster. However, as these tend to rust solid after a few years it is unlikely that you can either adjust out the 'droop' or that the adjustment is responsible for the 'droop'.
The bumper itself has a beam on the inside. There is no adjustment method between the beam and the bumper. If the bumper is not on the beam properly, it's toast. There's also no scope for movement in the mounting method between the bumper beam and the iron.
In conclusion, there are two reasons that the bumper has a 'droop'.
1. The bumper iron adjuster is out and in theory it could be adjusted back ( but is probably rusted solid ).
2. The bumper iron or the car's structure is bent.
I'd assume it is the result of minor accident damage either not repaired or badly repaired unless I saw good reason that the adjuster is out.
Hope that helps.