I wanted to get a Playstation card but it was obviously from 18 and I wasn't allowed to. But there's no age on the other cards and 15 euro is not a large amount either, if I'm allowed to buy a 14-year-old card
Yes you may.
I get these cards every day and I'm 14 too, but ps cards are not available for us from 18 so I have no idea ask the seller
You can do it. Neither Amazon vouchers nor Netflix credit is available from 18. PSN cards, by the way, the logo comes from Sony and has no meaning.
But even if you can and are even allowed to, a dealer does not have to sell them to you. Even without FSK / USK 18 approval, dealers can refuse to sell you these things.
Actually yes, nothing speaks against it.