I don't think it matters if they do or not. The next user can just delete the save file themselves.
Just as long as the game works, and it's not completely gross (like- if the previous owner got sticky crap or whatever all over it), then why complain?
I DO think though, it should be a common courtesy to make sure your game is CLEAN before selling it.
I hate scrubbing off old candy and price-sticker gunk off my games.
NOW, when it comes to POKEMON, it's actually kinda NICE when there's a pre-existing file. If you have another game and system, you could trade yourself some pokemon that you (starting a new game) can't catch yet. The previous owners might be farther into the game. It's nice if you want to get a good team going early.
I've never sold a game, but I've bought several used games with old saves. It really annoys me too. I hate seeing other people's progress on a game that I own. What was worse was when I bought my used 3DS. The previous owner didn't wipe anything. Not game data and not even the pictures. Luckily it was only pictures of a dog, but still, those are private pictures and just going to delete them felt like an invasion of privacy. People should wipe all their data before selling something.
What?
How is that invading? That makes no sense.
It's not invading AT ALL if the previous owners were just too careless to wipe their pics.
Common sense would tell them that another person is going to get their system. That in itself TELLS you that they'd see EVERYTHING you had on there. If you don't take the time to wipe the memory, it's your own fault. If you give it away with doing so, you're basically handing some stranger your stuff. KNOWINGLY. Unless you are just careless and stupid... but STILL- It's not the owners fault, it's not the seller's fault, it's YOUR fault if you don't wipe it. If you don't want anybody else to see your pictures, then you wipe it. Simple as that. If you don't, then you obviously didn't care enough, and the way I see it, it's your own damn fault.
And, also, I actually kind of like seeing stuff about the previous owner. It kind of makes it have that 'well loved' feel to it. (I'm talking about game progress) And, (as long as they're not BAD pics), if there are some pics of like- their dog, cat, family, or something innocent, it's actually kind of interesting. It's like peering into the life, even just a tiny bit, of somebody else. it's in a way, adventurous. But don't go around doing it. xD I mean, if they left it there, you found it. there's nothing wrong with that. Similar example- if somebody dropped a photo of their family in the woods when they were hiking and a stranger (to them) just happened to find it a few days later. Nothing wrong with it at all.
I don't see removing your save file before selling a game as important unless it for some strange reason holds personal information or explicit content such as a rude username (that could end up in the hands of a child).
That- That I can understand.
It would be courteous to wipe it if you DID enter 'rude words'.
Like how Prof. Oak's son is ALWAYS known as A$$bag or A$$hat, or $hithead, or some other word that the generations before would name their rival, out of sheer humor.