Only legit reason would be people who are trying to jump up into a signature campaign that is closed to lower ranks, and open to higher ranks.
That would in my book also be not legit, as you're buying an account to get improve your own "forum status".
Worth thinking about is also the reputation of the account for the seller. If someone sells a bitcoin account, you can never be sure what the buyer does with the account. He might post stupid things, or he might orchestrate a scam, that might lead the seller into being part of an investigation.
For example, if there was a new pirateat40, he might buy tens of account to prop up his ponzi, and once it all went up in flames and an investigation was launched, account ip's might be collected, and by using isp-registries home addresses extracted, and you might have your house searched and all your computer equipment confiscated as you could've been complicit in the crime. Unlikely? Not if the ponzi/scam runs in the millions of dollar range. All for making a few satoshi's. If the account can't be traced back to you, and you do not have any reputation that might be damaged, it's another matter, but still unethical.
Personally I think account trading should be denied, as I can't see what legit reasons there are for doing it, but I can see many reasons that are not legit.
