"Sticky" Zoom on Sigma 28-70

[conan24]

I recently sold a Sigma 28-70 f2.8 lens. It was a very good copy and never gave me any problems.

The person I sold it to says that the zoom "sticks" when trying to autofocus on anything closer than 2ft. They have tried moving the zoom back and forth in manual, but apparently doesn't help.

Help: Could have shipping caused damage? (I packaged it so that an egg would've arrived safe. well...just about)
Is it a lubrication issue? I've never had a sticky zoom before, so I'm not sure what causes it.

Any advice would be welcome. Thanks