The same reason you would sell any other social account. You build a profile that seems real, sell it, then the buyer uses that profile to upvote and post comments on their new product release which blows up on HN and generates much more interest and potential revenue than it would otherwise.
I don't have proof that it is happening but it happens on every other social site so it would be shocking if it isnt happening here.
GDC7 was created 62 days ago and has two-digit karma, sure you're not thinking of someone else? (Just seems unlikely you'd recognise such a new and not particularly highly up-voted account.)