A list of Proper Instructions how to properly sign a BTC sent Transaction could be most useful to all!
brainwallet.org has this ability. click the "sign message" tab and enter the private key of your payment address. once your message is typed, hit the "sign message" button. then, paste the EXACT message you typed, along with the signature, into a pm.
you can also verify someone else's message in the "verify message" tab. try it out:
Message:
this signed message is proof that i have the private key for 1HFn2U71FJv7MQfbXkVNtR7S7j9h39EVFJ.
Signature:
HKrNm8daO6JeMpEV08+yaY5EU1Uf438xPSyaf2/Es8EsyfCUptTaaOhIG2pKHxA6pEZM8CFoLODFyPgdEL+1IKg=
A) So you have to put your private key into a website somewhere? That sounds risky
C) Perhaps making a burner blockchain.info account would be a more secure method?