While I can’t find the official resource that the completion of purchases is impossible, I am 99% sure that it is simply impossible. All you can do is redirect the user to the payment page using this template for the URL:
http://payments.${DOMAIN}/ws/eBayISAPI.dll?CheckoutProcessor&item=${Item.ItemID}&transactionId=${TransactionID}
Where DOMAIN can be, for example, "sandbox.ebay.de" or simply "ebay.com".