Basically, if you're doing a model edit then you don't have to own the base, but THEY should if you're editing for them. Pretend as if you're editing their copy of the base.
For Example: They buy the base, send you the files for editing , then you send back edited copies.
This is so they don't pay you $50 for a retexture and then expect you to spend half of that on buying the base. As an editor, you shouldn't have to be paying for the base unless you're going to use it yourself!
If you're making a new model using parts of the body or something, then you should buy the base.
For Example: Using the body for a commission where you make a new head and textures.
Mainly just don't edit the model slightly and then sell it as a new base or something. At the end of the day, I don't mind what you do so long as you're not just reselling.