It is an app that basically acts as a classified ad list for people to sell things.

An anonymous person can list an item to either be given away or to sell for a price.  They list their item on the 'Item' form.  

If they list an item for free, there's no cost and no need for Paypal.  Here the field 'Asking_Price' in the Item form  = 0.  The item will be listed for x Months where the user specifies the number of months.  The form is submitted and the item is listed in the 'Items For Sale' view.  

If they want to sell their item, where the Asking_Price >0.  Say $5.  In this case, they need to pay a rate of $1.25 per month in 'Listing_Rate' field.  This rate is the same all the time.   The item will be listed for x Months where the user specifies the number of months.  The total amount to pay is Months * Listing_Rate.  NOW I need Paypal.  Do I need a separate Payment form?  The Item cannot be listed until payment is made.  Once paid, the item should appear in the 'Items for Sale' view.  

I cant figure out the flow of this process.  Can you help?