Is it safe to give out serial number?

Not without registering it with Apple first. Then when you make the sale, just remove it from your Apple account so the buyer can register it under theirs.

If this is about checking an AppleCare warranty, you can transfer the warranty after the sale has been made.

I can't really think of a reason not to. If its registered they can't re-register it -- even if they did register it, it doesn't really seem to mean anything and you can contact Apple to rectify it.

In the end they can't really do anything with it - it is really just for allowing police to identify your product if its stolen, or for AppleCare tracking.

When I am in the market for a Mac product on CL the first question I always ask is for the serial number. I say I'm just wanting to confirm the warranty, but the main reason I ask is to call the police and ensure it's not stolen. If someone is hesitant to give it out -- I figure they have something to hide (stolen?) and just move on.