The main problem with doing that would be slightly personal: you give access to a stranger to your account name and password, as well as potentially your credit card information (or a way to buying digital content with your credit card), if Sony is storing that somewhat indefinitely somewhere...
I would say there is too much risk associated to do it. I personally wouldn't do it.
I have a few US PSN codes to sell for a reasonable price (cheaper than pcgamesupply). PM me if interested, 1st come 1st serve basis..
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