You need to know that your upgrade can be withdrawn at any time before the flight. What if the person you traded with got the upgrades from a third party that he doesn't really know? You also need to know that the agent at checkin might ask you about your sponsor. What are you going to say?
From my perspective I would caution you to get to know the game before you jump in headfirst.
Trying to get to know the game better here per your wise advice.
So if I purchased a SWU on eBay, and so far the transaction seems smooth and successful, the upgrade is confirmed on the airline website, something can still go wrong at checkin? What kind of questions might the checkin agent or GA ask me about my sponsor?
Thanks in advance, and for all the past advice I've gotten lurking for several months!