Don't know if this might be of interest we setup a password protected RSS scripts directory to sell our RSS related PHP scripts and it works quite well. Drop me a note if this might be of interest and I can tell you how we did it.
I suggest starting off by running a google search for a particular php script. You'll note that some of the results lead to websites which offer free/paid advertising for script sellers. You can set up a website to sell the script on and start promoting it on script directories. Good luck.
Depends on the script. You can host it and sell it yourself or try some forums like DP. If the script is good I'll try the first option. You can release it open-source and earn from donations and advertisement.