Yup the PHP market is very highly competitive, specially due to all those free softwares out there. What ever you put together, it should be feature rich enough to make the customer buy it and not download another free script.
That means your software should be easier to install and the support you provide must be quick and efficient.
Try to avoid saturated categories such as CMS, news managers and ofcourse bulletin boards
Give it a shot with free scripts and then move up to commercial stuff.
Good Luck!
Luthfur