The first step to setup paypal adaptive will be to create a developer account at https://developer.paypal.com/

The correct sandbox url is https://api-3t.sandbox.paypal.com/2.0/ and the test Application id is "APP-80W284485P519543T"

Paypal charges everyone who receives money. For subscriptions you can setup a different gateway such as Authorizenet. You can select the one to activate for subscriptions from

.../admin.php?p=payment_services

Complete the paypal adaptive form with the your sandbox account details at

 
.../admin.php?p=payment_method&mode=update&pid=362&vendor_box=0

NOTE: In your sandbox you need to create

1. SANDBOX EMAIL ADDRESS FOR MALL OWNER WHO RECEIVES THE COMMISSION
2. SANDBOX EMAIL ADDRESSES FOR VENDORS TO RECEIVE PAYMENTS
3. SANDBOX EMAIL ADDRESSES FOR USERS (BUYERS)

To go live with paypal adaptive you need to apply for your own APP KEY. Paypal support will guide you through the whole process.

If you have paypal specific questions, the best will be to contact paypal support they will answer all your questions.