The key is finding the right spot in the lot where that can't happen. Even if you have to walk a bit, park on the end. My last 2 cars had no marks like you mentioned front or back. I even had the last car in the City of Chicago for a year and a half.

Buffing the bumper will involve someone massaging out the clear coat until the surface scratch is no longer visable. If the scratch goes down into the plastic, it would have to be repaired, sanded down, and could be airbrushed / matched to the rest of the bumper. Replacing the bumper for a scratch won't happen. Not even necessary. Worst case, it would be stripped all the way down, reapired, and resprayed. Not necessary if you know of a good body repair guy who can blend w/ an airbrush.