Unless you need something unique that never has been done before I would probably start with one of the existing scripts to use as a base.

Than you can always adjust it to your personal preferences...

Yea just as skinner said.
Although it will take a bit more of your time, your end result will be more original.
Alternativaly you could always buy a old script and build off it, many people do that and then sell it at a higher rate.

Depends, some times I do, but for big projects no! I always hire someone!

:S
It also depends if you got the time and really want to script for weeks!
But i guess if you got the buget then yea.

Another factor that could make you develop your own script is if there one on the marketing do not meet your needs.

Indeed.
Personally i think there are a few scripts on the market, that you would think of that you cannot find, leading you to make a scripter code it for you.
Like i said before, there are not alot of scripters online, making them make so much money, its incredible.