UA and AA both have excellent programs for international upgrades. You should concentrate all your flying on one carrier and if necessary do a couple extra flights to achieve top tier status. You'll then have your own upgrades for your international flights - as well as having higher status, thus more easily clearing the upgrade list.

I would be very cautious about buying upgrades from eBay. If it has worked well for you, you are fortunate. I was talking to a friend yesterday, who happens to be an airline rez agent. She was speaking of cases where her airline has pulled upgrades when they have determined they were purchased on eBay.