So I tried to make a trade shop here is what I did:
[Trade]
20 mushroom
1 grass

But I figured out that they had it so mushroom was not a item...

 

Block or entity? Either way google the item id for what your selling and put that in place of the item name.

 

It was a entity, and I do not really understand what your saying...

 

Look up item ids and there should be an index of them

 

I can look up the block id for you, what item exactly is it?

 

He/She is probably thinking like...

Anywho. What mushroom are you going to sell? Normal or red? I will link you the pages for them.

Mushroom – Official Minecraft Wiki

 

How long have you been waiting to use that meme?

 

A long time man.... A long time....

 

The normal mushrooms.

 

*The red enities*

 

They're both normal mushrooms

LOL