Yes, it is what is usually done. But it has certain limitations including you cannot use an address you can't sign but still receive and send funds from/to.
Perhaps I misunderstand you, but isn't that exactly the point. If the escrow signs a message from the escrow address then it shows that the escrow does have control of the funds for the escrow account and can spend those funds to the other party(es) upon fulfilment of the terms.
It proves that the Quickseller is in control of his account. He cannot later claim it to be hacked or anything.
He is establishing that he is acting as the escrow and is reponsible for the funds.
Ok, I guess there's some value in this.
Escrow's usually sign the message before receiving funds.
That's not what I'd do if I were escrow. And it's not what I'd want to see from an escrow I was working with. It's one thing to say, I'll be escrowing this deal. But it sounds absolutely nuts to say "i gurantee this deal" before receiving anything from either party.
Escrow guarantees the payments. In case the amount is lost due to his fault, he might need to recover it from his own money too. As the case may be. Signing message does[not imply receipt of funds.
Signing a message from a bitcoin address with funds on the blockchain does imply control of that address, and therefore, the ability to use it to pay out upon completion of the terms.
Obviously the folks on this forum can trust who they like to and do what they want to do. I'm not calling anyone out here as shady or untrustworthy. FWIW, I just figured that quickseller signed using PGP rather than the escrow address by force or habit or something. If he
is guranteeing an address he doesn't control, it would be pretty surprising. But maybe we should just ask him directly:
Quickseller, do you control 1NPDJ8gmNbZd2QimaQzbFBmGUtCX5djgrC ?
I'm quite surprised that what I said is in anyway controversial. It seems quite straightforward to me.