If you're going to put this on your blog, why not call some local solo or boutique firm TM or copyright attorney(s) to see if they'll give you a quote or two on the topic which you could use with attribution (or see of some of the forum's attorneys who provide their real names and practice details will give you a quote)? If your blog has enough of a following--and even if not, they might be happy to do it and it would make your article more credible and authoritative. What anonymous posts like mine tell you here could be totally wrong, for all you know.*
That said, OMG IP is right in making the point that people don't have to ultimately have liability in order to be sued.
As to ultimate liability with regard to trademark law -- any liability in trademark will actually PRIMARILY be with the business USING the logo. The standard for infringement is whether or not there is a likelihood of confusion as to the source of the goods. If people are likely to think products/services bearing the purchased logo were related to the the company who had the original, copied logo -- even if only similar and not identical -- then there's infringement.
If this is done innocently, that will mitigate penalties, BUT if the business purchasing the logo didn't bother to do a trademark search first, that works against any claim of innocence. In any event, however, innocent or not, as you point out, the second user still won't be allowed to continue with use of the logo (unless they can get the party with priority to work out a deal as to a disclaimer or license).
With regard to copyright, the person MAKING a copy is the one with primary liability for infringement, but if B hires A to make a copy for B, that definitely implicates B in contributory infringement.
Am not well-versed enough in copyright to know without further research to what extent innocent possession and use of a copy made by someone else could lead to penalties, but here again the law takes a dim view of business people who shut their eyes and claim they didn't see the problem.
When it comes to whether or not someone did something "innocently", the law usually is concerned not with whether or not someone had "actual" knowledge of the wrongdoing, but whether they "knew, or should have known". In other words, you can't say you "didn't know" if you failed to investigate where a normal, prudent business person would have investigated.
Best of luck to you.
K
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*Note that while this site is meant to be a forum in which attorneys can discuss IP issues with colleagues, and has many attorney participants, regular participants include both attorneys and interested lay people -- some more knowledgeable than others. No one has to show bar registration or demonstrate competence in IP law to participate. While some attorneys do use their real names and provide links to their practices, so you have a means to verify their status, many others, like myself, participate anonymously. So, e.g., while I am an attorney with over a decade experience in IP, you don't know if I'm lying when I say that. The only way you can tell if I know what I'm talking about is by using what you learn from my post to do additional research into more authoritative sources and see if they agree with me.