Is a license required to develop a mod you don't intend to use personally?

[Destriarch]

I was thinking of developing mods for VB as a sideline, but the licensing price is rather high for me since I personally have no use for a bulletin board. Is it necessary for me to purchase a license in order to develop mods on the behalf of somebody else if they already possess a license, or is there some way I can use their license to develop a mod for them?

-Ash

[vijayninel]

If some licensed user allows you to use their beta forums for developing mods then you can do that.

[Marco van Herwaarden]

If you can develop without a vBulletin installation then yes. You can also use your customers development installation.

[Ricky Doherty]

if they already possess a license, or is there some way I can use their license to develop a mod for them?