Elenarie: Nobody guarantees that your pizaa won't taste like shit, or your lawyer fuck you up many times over, or your construction workers masturbate in your bedroom while they work...
... yet you will pay all of them.
Rincewind81: When my Pizza tastes like shit I get a refund or a new one, if my lawyer makes a mistake he has an insurance and I don't have construction workers. If they want to monetize mods - fine. But with this move they have to offer a guarantee, that they will work in the future and with other mods...
Doesn't sound like anything different here, except you have 24 hrs to request one.
Q. Can I get a refund?
A. If you discover that a mod does not work for you, or does not meet your expectations based on the description of the mod, you can get a refund within 24 hours of your purchase. You can view the full refund policy here.
None of this should be surprising to anyone. Valve already saw how mods can be monetizing with TF2/DOTA/CS. I also don't think this will bring the free modding community down. If no one is playing your mod because nobody is buying it, that's not a good thing for you if you are a modder.
I think what people here are afraid of isn't that people can sell mods, it's that they know people will in fact buy them and there is a potential market for them.