WE building question

where do i find the mercenary/item buildings that you can buy things from; IE with item slots?

They're in the Nuetral passive section.

any builing could sell mercs/items. give them sellable units/mercs and the hero choose ability

ok, i have a goblin merchant in my map and it sells a bunch of crap that i dont want, how do i change it to sell what i want?

Where it says "items sold," either delete those and put in the ones you want, or select an item and press "edit item," to where you can replace it without having to delete and then restock.