An idol shop would create idols of existing gods. It would not invent new gods themselves, but might add new idols to their collection as new gods came to be worshiped in the area.

" The designs for the idols sold there had been passed down from father to son for several generations. Though each added some new idols – the historic ones always seemed most in demand. "

Reference:

Rabbi Robert B. Barr "Abraham and the Idols" ( https://www.ourjewishcommunity.org/learn/midrashim-legends/abraham-and-the-idols/ )