Nice ^^, anyone know how this works or anything to add? ^^

This!

I'd say it's more like going to see a movie, not liking, and trying to sell the viewing of the movie.

Or not liking a recipe, so selling it to get rid of it from your brain.

thats what i mean Teaching Npc spells and you get coins for it !

Thats a better example

Because you get as much money as you like and still get to keep your spell?

Well, you can't sell something from your memory.

You can if you're selling knowledge.

It could help for ally quests if you could give them equipment and teach them spells, especially if you do it at a higher level and they're a bit underpowered. "Your equipment: heal and invisibilty. Now go stand over there and keep out of trouble."

Maybe selling them for money could cause a problem, if you could just go around teaching your whole spell list to everyone in Skyrim willing to learn for easy cash, and there's not even a Mages Guild to come after you for unsanctioned training. Unless there were unintended consequences to doing that like you come back to town and find it burned down from every citizen showing off their fireball spells.

But gameplay-wise, you could just roam around teaching people spells and getting essentially free money.
I think the OP meant 'forgetting' a spell and giving it to someone else, but that makes no sense lore-wise (or logically).
EDIT: nevermind, nakanja nailed it.

You shouldn't be able to teach spells to anyone, neither to sell your equip to any shopkeeper. Not everybody would find usefull a undead cowering spell and a smith should realize that if he buy your daedric claymore, won't be able to shell it again ever. Of course, giving some logic to this would be difficult.

As far as I understand it, you will be able to train other npc's, not just the other way around. You might be able to train a companion to use a particular spell for a price, or it might be part of a quest to train another in a certain skill for a particular job. So I imagine that you would be able to sell spell in that way.

Although, I can't remember for the life of me, where I heard this, so I might be wrong. So feel free, anybody, to correct me. I'm not saying I'm right. I'm just saying I think I heard it somewhere or read it somewhere. But if I'm wrong, then I'm wrong. But it would be cool if they did do that...

I think that it could be made that way so it can give you some extra money every now and than and that you can't actually live out of it by making it so that you can get a student only every now and then and that money earned is not some huge amount.
The point is that it's not problem to balance it out.

Sell, no
Forget spell , YES!

When I have a heal damage 50 point for 3 seconds spell I dont want to keep my heal 48 for 2 seconds or 24 for 2 seconds :cryvaultboy:
Unwanted clutter

I agree. It would be really cool if you could create a new spell (similar in a way to Oblivion's system, but maybe with more variants or new uses for spells?). I mean, npc's sell spells, why can't you?

Selling a spell doesn't make any sense. You can teach it to someone else for a fee, but why would you forget how to use it in the process? A system to hide unused spells would be nice however.