Hey!
I don't think any database is required in your case.
As you said you will be selling your services
So what you need is >" Payment Gateway System"
which banks will provide.
Yet,having record of persons visiting you website ,and interested in your services will be nice idea.
1)It will help you keep a track on who's gone for your services and for what amount.
2)It will let you contact them in near future (get email while registrations).You can have better CRM.
3)It will let you have clarity at your side with records.It will be helpful in case of migration to other technology too.
4) Although records will be available to on banks payment gateway system.Yet in case of any inquiry you can get info. from your local production DB at the earliest.
In my opinion you need to have something like JOOMLA
which maintains everything in same package and makes your
job easier.It has in built MYSQL.
get tutorial by typing "Joomla tutorial" on google.
I need to know what exactly your reqmt. is ,in order to suggest you any further alternatives.
Hope this will help
Cheers:)