Also not all online services work that way, BlockChain.info's MyWallet for example works exactly like a desktop wallet, while InstaWallet functions as you say. ... Several sites/services that REQUIRE the user to be in control of the sending address exist and have been very clear and don't seem to have an image problem.
It is wrong to assume a user has access to a sender address. I do not care if some services work that way. It is a false assumption and will cause problems in the future.
I would still say it is a better policy than just taking the coins and saying 'oh well... thanks for the donation'.
I do not understand how that is better. Let us use the example that started this conversation:
https://blockchain.info/tx/dfb30e8dd29628b3ee14ff6b0b2d9946845a919b240cefffc2ebeab6327e92b4In this case one of the addresses is actually a donation address. But what if that was not the case? Which address do I send the refund to?? Any of them?
One reason I am so adamant about this is because it is very possible for this to happen completely out of the users hands with many small inputs. If their client doesn't do the right thing, or even if it does some mining pools will deliberately prioritize large TXs with small amounts.
I agree completely and want to be fair. That is why we are discussing this. But I do not want to send Bitcoins to some address that may not be accessible for the user.
That is bad policy.
Perhaps it is easier to create a "lost and found" on nastyfans. It applies to each member individually. Then members have time to decide what should be done and where to send the refund. For each lost item a text area can be available for the member to enter instructions.
This does not clarify anonymous Bitcoin deposits. If suddenly 1000 BTC land in a nastyfans address I still do not know what to do. Maybe a general "lost and found" where we can use a poll to decide what to do.