how to verify that someone sent money to a third party

[je1p7u7]

I'd like to check with someone more experienced if my thinging is correct.

Let's say Alice paid Bob for a GPU via BTC. I would like to repurchase the GPU from Alice, but I need to be sure that she bought it Bob.

That said, is it technically and otherwise possible that I request Alice to sign a message that would prove that her wallet address sent funds to Bob's wallet?

[MartinReynolds]

By going on BlockChain.info and typing in their BTC Address and seeing if they recieved.

[je1p7u7]

I don't know Alice and I don't know her wallet address. I guess this question boils down to confirming that someone is the owner of an address which is definitely doable.

[MartinReynolds]

That's why you confirm them before you send.

[BitPappa]

By going on BlockChain.info and typing in their BTC Address and seeing if they recieved.

That is really good info for a newb like me!

[Maged]

That said, is it technically and otherwise possible that I request Alice to sign a message that would prove that her wallet address sent funds to Bob's wallet?

Yes, but only if the bitcoins weren't sent from a shared wallet like MtGox. If they were sent using a wallet like Bitcoin-Qt where you control every private keys for your addresses, Alice might be able to sign a message using an address that was used in an input to the transaction with Bob. I say "might" because in Bitcoin-Qt the addresses used as inputs to the transaction might all be hidden because they were change addresses.

So, it is possible to say for sure that Alice paid Bob, but the inability to sign a message to that effect does not indicate that you aren't speaking with Alice.