What do you mean? It is possible to reuse addresses depending on the wallet. However you do not need to care about that, all you need to care about is whether the seller has any funds at all. Signed message is the only way of verifying that someone holds private key for that address, and therefore can spend the funds from the wallet.

You don't need to use the blockchain wallet, just the block explorer. You can also try Chain.so as i like it more and find it more useful and working than blockchain.info.

Is the seller on bitcointalk?