They hide it because some people a while back would take the serial number from the listing and report basses stolen when they weren't actually stolen, even presenting false receipts at times and causing all kinds of pain in the ass because suddenly the owner was expected to prove themselves for some reason. Jammed a number of people up in the forums from what I read, and the blurring serial number practice has stuck.
It's not that likely but still, too many musicians do weird shady stuff when they think they'll get gear for free.
An interested buyer might need to know part of their serial number for year model purposes, but unless you own the instrument the rest of the serial number isn't something one needs.
Unfortunately theft is also always a possibility with any instrument though theives probably usually don't showcase a serial number at all.