At first I thought that question was from Chris of the EU blog lol.
1. I haven't sold that many shares but when I do I only sell here. Like you said , it is really hard to keep track of who is the owner if it ever gets compromised. Though I can understand why people sometimes sell on other sites as that particular share may not get interest here but could sell somewhere else. We cannot stop people from doing that nor should I want to force someone not to but I hope that everyone tries to keep shares bought here stay here.
2. I think the seller should think long and hard who to sell to. I'm not saying they can't but if the accounts get compromised (because of selling to someone shady or newbie), I really think that the seller should reimburse the other shareholder. Sometimes though they could genuinely think that person was trustworthy so I don't think we should enforce it. That is why I always try to host when possible as I will usually keep all my shares for a while.
One thing that I started to do now that I saw others do such as Realit3y is add something on the info that people will instantly know that it's you. For him it is the number 3, for me I just put pancakes somewhere in the email.
To keep it relevant share-wise (only to trusted CAGs) selling:
Sam and Max(both series) and Jurrasic park for $4
Crysis 1 for $2.75
yes I like to put 3 in the mail and put fucked up passwords that are non-sense most of the times, but that not mean that the 3 will be there all the time.
What I have seen done by someone else sometime ago is that on their wall of his profile he had the email etc, and I think he had the resells in which the ppl was with at that time.
Might be only relevant to be done if you are selling with CAGs but also most of the time ppl once they sell their resells to someone else won't be talking to the host and keep track of the resells of who owns what.