It depends. I think most sellers only have one or 2 dress forms or mannequins.
I have been selling online since 1998 and have 3, oh wait, oops, I have 5, but only use 1.
(A 6 ft mannequin, a dress form, a suit dress form for men, a half woman form and a half mens form.)
The one I primarily use is a ladies modern day size 6 full mannequin.
I try to adjust the clothes to fit with clips, crinolines, padded slips.
You always need to give the measurements of the garment and it's really a good idea to show the garment not on a form, especially for larger dresses so shoppers can see the true actual shape of the garment.
This reminds me - I really should get rid of the other 4 I have.