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What if I wanted to send some coins from several addresses and then send their balances to one address which I don't own or control, should I simply send private keys to the buyer of my coins while he gave me an address and I have no means of communicating to let him know and even if I did then he wouldn't put his trust in me knowing I have the private keys.
Here I simplify it for you:
I have 0.4 in address A, 0.4 in address B, 0.45 in address C and trying to sell 1.2 to address D which some body gave it to me to send him 1.2 to him and is waiting for me to send him 1.2BTC.
When I'm using Bitmixer I could send 3 transactions and the extra 0.05 I consider as fees, so I send Bitmixer 1.25 and they will send 1.2 to the address D.
I honestly don't see what the problem is? Your big beef with chipmixer is that you don't think you can send to some given address because all you have is private keys and supposedly no way to contact the person you're dealing with? (one wonders how you setup the deal in the first place?
)...
Anyway... Ignoring the fact that this seems like a fairly "edge case" scenario, your argument ignores the fundamental rule of Bitcoin that if you have the private keys, you can control the coins.
Why can't you just execute a sweep transaction that sends the contents of the private keys provided by Chipmixer directly to the "Mystery Man" waiting for his 1.2?
There is no rule that a sweep must go to an address in your wallet...
And just in case that is not clear, I'll simplify it for you:
So, send the contents of AddressA, AddressB and AddresC to Chipmixer... they give you a bunch of chips... because you're such a swell guy you "donate" 0.05 BTC worth of chips... collect the keys for the 1.2 BTC remaining... and then "sweep" them to AddressD...
All this kind of reminds me of a common saying:
Better to remain silent and be thought a fool than to open your mouth and remove all doubt