This is where the public place comes in to play. Not only does it make sure you won't get robbed at gunpoint, but you can also get help fast if needed. If you have to, then have the person meet you in a police station or some public place within a walking distance of one. This will insure that the police can arrive there as quick as possible if needed. If you're selling a very large amount, carry a taser. If he starts to run away, hit him with the taser and call the police. I'd be more worried about someone trying to rob me at gunpoint than I would be about them running away right after sending the funds. You could catch them rather easily. If they get into a car, get the license plate number.
When you are trying to have a transaction to the other people you met through online better while you are having a conversation to them get some validation of information as a proven to have a legitimate transaction most of the common problem today is no information of the other party wants to transact if you don't want better to make a meetup transaction to verify the items or get some midman to handle both things and make sure the security of the traders.
^ this 100%. Whenever you're dealing with someone its always important to know who they are if possible and understand their background / have knowledge of their background if you can.