Regarding Facebook I have mine set up pretty much as private. I don't allow friends of friends to be able to access me or ask for an invite. I post pretty liberally on there but the only one's that can read what I have is if I opted for them to be able to access it.
As for eBay I've been on there since it opened it's doors labor day 1995 and it is a safe place to be if we follow the rules they have in place to protect us. No one on there can contact me unless they have a transaction going with me via eBay. I NEVER WOULD start or continue with emails outside of eBay with anyone no matter what. IMHO it's a place of business and not for personal use and should stay that way.
As far as stalking goes back in 2005 and 2006 I was stalked online for those two years because of my relationship with a direct selling company. One of the gals in the company did not like me at all and she made my life a living hell. I had to first go to my local police with all documentation and file against her. It was than forwarded to her state and the detectives there took charge of it. By the time they were done with her she pretty much disappeared off that companies website, on facebook and the yahoogroups and such I was on.
Personally there is no way that I will get all that friendly with folks online outside of groups I belong to and if I did that person would be checked out before I communicated with them via private email.
Regarding the eBay incident mentioned here by the OP I would recommend you report it to eBay and make them aware of it. If he did it to one he'll do it to others.
Regarding eBay I won't deal with a seller unless I check them out first by pulling there user info and making sure they are who they say they are. In the last 16.5 years being on eBay I've only found about 5 or 6 who were not who they stated they were after checking there user info.