If it is your own work, as in you drew it on paper and scanned it, then took it into Adobe Illustrator, InkScape, or any other vector producing program, sure, it's fine to sell it to one person only. But if you're taking others' works and selling it, and they didn't give you the right to do so, then they can take you to court for what copyright and intellectual property (IP) rights you've violated for the country your are an inhabitant of.
You can only sell a logo once because whoever buys it will end up trademarking it. Can you imagine one party successfully suing many others because you went ahead and sold it to several people?
Since I see it is an extremely complex topic, because then nobody might do designs with corporate marks unless previously he asks the designer of the logotype for permission which is clear that it isn't like that
If someone uses the Nike logotype I am sure that he isn't going to ask for permission his creator or the proprietors of Nike for it
On some pages they companies insert dozens of logotypes of completely foreign marks, and I am sure that also it is without authorization
I believe that there must be some concrete law on the use of logotypes (when they can be used, when not, and with the permission of whom)
Moreover, most of the big companies have in his web the logotype vectorial managerial to download so that the designers could use them without distorting the corporate image, therefore I understand that it isn't also necessary to ask them for permission (although the fact of already selling it implies other questions)
I understand for the expressed opinions that sites as http:// www.civicheraldry.com /
they would be illegal, and sincerely I don't believe that this way it is
What happens if instead of selling a vector logotype of a company, I sell the service of looking for it in the web, download it and delivering (email) it to the client? also would it be illegal?
For me it would be the same... although it is possible that legally the implications are different