Why not Make your Own Parking Page? Few Q's on Selling

I want to park a few domains I own and Im wondering why I just cant create my own parking template and place my own ads into it?

What is the point of using a parking service? Doesnt that cost to use a parking service like godaddy and others?

The reason I ask is because I have some local domains that I am selling to local biz owners and I want to make a custom parked page, insert my own ads (links to their competitors), and put a for sale button on it.

Then my next question would be:
Would I be able to sell this domain with a paypal 'buy it now' button?
What would be involved in that? I assume once they paypal me the money I would then need to transfer the domain over to them? But dont some hosting companies have a certain number of days that you have to own a domain before you can transfer it?

Im a little confused here and am hoping someone with more knowledge on domaining can help clear this up for me.

Thank you!!

I can't speak to the benefits of using a "parking service" as thats how I stumbled onto you post. [looking for info on parking services]

You certainly could make your own parking pages.

Props on "incentive'izing" the local biz to buy the domain by advertising the competition. Had much luck with that?