You can look at all the post that are closed for selling gift cards or psn cards.
It's illegal pretty much in every sub with the exception of "Steam" and "Battle.Net" sub. The reason being is because it's very easy to gift these things and a trade history to prove you traded it.
On PSN there is no history and borderline impossible to tell who scammed who if one occurs. You could sell an expired code, or the buyer could claim you gave them an expired code and there is literally 0 way to prove who is right. So therefore the site banned it for most subs.