Does anyone use multi-sender transacations?

[Murphant]

I know multi-sender transactions with sending addresses that belong to different people are possible in theory in bitcoin, but does anyone actually use them? Many analyses of the bitcoin network assume that they are seldom used, I'm just trying to get an idea of how much that is true in practice.

[Foxpup]

Yes, and in a big way. Virtually all web wallets use them. Eg, suppose Alice, Bob, and Charlie all have an account with the same wallet provider. Alice and Bob deposit BTC5 each, and Charlie deposits BTC10. If Charlie then sends "his" BTC10 somewhere else, it might actually come from Alice and Bob's deposits. It's exactly the same as if you deposit $100 cash in your bank account, and then withdraw it again later: the bills you get almost certainly won't be the exact same bills you deposited, they'll have likely come from another depositor, or even several different depositors. The assumption that all sending addresses belong to the same owner is bogus if web wallets are involved (unless you consider the web wallet itself to be the "owner"), and so is the assumption that it's safe to send a refund back to the original sending address, since that address might not actually belong to the person who originally sent the money at all.

[Stephen Gornick]

I know multi-sender transactions with sending addresses that belong to different people are possible in theory in bitcoin, but does anyone actually use them? Many analyses of the bitcoin network assume that they are seldom used, I'm just trying to get an idea of how much that is true in practice.

Did you mean multi-party M-of-N (BIP 11) transactions?
 - http://en.bitcoin.it/wiki/BIP_0011

[Murphant]

It's exactly the same as if you deposit $100 cash in your bank account, and then withdraw it again later: the bills you get almost certainly won't be the exact same bills you deposited, they'll have likely come from another depositor, or even several different depositors.

I had though of that but somehow had not thought properly of the consequences for someone who is following the money trail. The trail does get scrambled when thought of this way. Thank you for enlightening me.

Out of curiosity, do web wallets usually create one address for every user or do they have one or a few big addresses every user dumps his bitcoins into?

Edit: answering my own question, both ways seem possible, the latter especially with hybrid wallets
https://en.bitcoin.it/wiki/Browser-based_wallet