When someone sends a payment to my address, I need to find their primary address, the one they control. But it's obscured by all the output/input addresses. I can't figure out which address is the one they control. I've looked on blockchain.info but all I see are several input/output addresses.
How do I know for sure which one it is?
There is no such thing as the "primary address". Transactions consts of one or more inputs and one or more output. You should not assume the sender controls any of the inputs as they won't in a shared wallet (exchange account, eWallet, etc). However if you feel the need to ignore this warning among the inputs none of them are more or less primary than any of the others.
