I'm working on a multisig ebay.. All open sourced. It looks awful at the moment since it uses old bootstrap and I'm not a particularly good UI developer.. working on improving that lately. Basically buyers give a pubkey per order, vendors keep topping up a list, and the admin has an electrum MPK there to continually generate order pubkeys, and addresses to receive payments for registration on.

I suppose it wouldn't take much to modify into an exchange, but at the moment it doesn't consider unconfirmed transactions at all. I wonder would people use an exchange that took a full confirmation to get your funds since, it's all on the chain?

At the moment it assumes the users will sign for themselves and paste the tx onto the site. Hoping to improve this once I clear some stuff off my todo list..

Javascript would be a nice solution, using BitCore to generate BIP32 keys from a passphrase and the necessary key for signing. But people don't like JS at all. I'll probably do this for my own, but it complicates the issue of allowing buyers/sellers to communicate and restore the funds for others.

Otherwise, accepting a BIP32 extended public key from users would be great. God knows how long away that will be. Or even, clients issuing BIP32 keys to anticipate the need for recurring transactions/pubkeys between people, and for better support for multisig. I'd love a QR scan/bitcoin URI for pairing/signing a multisignature address, it's something I'll be working on after a mountain of other work.