This is really annoying, why does it happen?

There's a 10% commision tax on all sales. If you sell for free, you go into the negatives.

@Venn: ah, didn't know that. Well that demotivates people to give away free things :(

Put it up for 1 mushroom if you don't want to get -1 upon selling it for free.
Nothing is stopping you from sending the buyer 1 mushroom back(though this technically wouldn't be any different in the end than selling it for free as you'd still be at -1 in the end because you get 0 mushrooms from 1 mushroom sales)

You could sell for 0 apples, as there's no commission on that

^ That's true