That is a great question.
I was wondering that myself when I heard they were going for big bucks on e-bay. I mean, my train of thought is you need to forfeit your account essentially right? I always assumed that was why they were fetching thousands of dollars.
That is, your not just getting additional value for PT but your selling your PSN account and ALL digital content. Which then enters the realm of "strictly against sony's terms and conditions" hence this type of thing became a craigslist affair.
I am certain there is NO WAY to transfer ownership of digital property from one account to another so no matter what work around someone cleverly drafts up, you are still in violation and running a risk in the event that you can still keep your account.
In short, I wouldn't do it UNLESS I was completely comfy selling my account and even then it would have to be to someone trusted as I'm certain the potential for something fraudulent occurring thereafter exists far beyond what I know of/could predict.
Sony is quick with bans when it comes to funny business. I don't believe there is any way to do this without rule breaking being involved. Thus, I simply wouldn't do it. I'm sure that wasn't the answer you were looking :( Sorry.