You can't get locked for picking up free stuff. If it's on the ground and not on someone's estate then everybody can pick it up. So why shouldn't you? I.e. click on it and try to pick it up. If you can't pick it up you'll see a message box saying so.
Of course there are places like the memorial where you shouldn't pick up stuff as those are meant to be gifts and not freebies.
Another thing is players giving away free stuff. If it's cheap stuff then you shouldn't have bad feelings. Though if you are about to get expensive stuff like a free Mod FAP, then you should be alerted. I'm saying it's likely that you're not dealing with the original owner of the avatar but with someone who hacked that account. In such a situation I'd thankfully accept the stuff but I'd take screenshots and send a support case to MA to look into that. Whenever the original owner is back online you can always give that item back. I think that this would be a nice gesture.
Though, if you're selling that expensive item again for a common price, you make your hands dirty. I don't know what could happen since MA's motto is "all treads are final", but it should hurt you in your mind.
Also, if you scam somebody to get something for free or damn cheap, then you're the one who may get locked.
