Thanks for the suggestion; we'll add that to our wishlist as a possible new feature we might consider implementing at some point in the future.
The buyer doesn't need to provide their Transaction ID; you could just look up the buyer's name or email address in your E-junkie Transaction Log and obtain your Transaction ID or the reg. key issued for their order there. Moreover, PayPal issues different Transaction IDs to the buyer and seller for the same transaction, so if the buyer provided the Transaction ID they found in their PayPal account history or receipt email, you wouldn't be able to find that in our system, as we always reference the seller's Transaction ID.
We could suggest using our standard Integration method to have the order details (including the reg. key issued to the buyer) transmitted to a custom script at your end, which could log that data and do whatever you wish with it, such as providing your own way for the buyer to obtain their reg. code or other details. If you use our Send Generated Codes feature to deliver codes generated on-demand by a keygen script at your end, that would inherently transmit all the order data to your keygen script just the same as our Integration feature does: