you need to move all of those coins to new addresses (that only you have keys for) before the seller does.
You are correct about that. Our methods of doing business are more complicated than what I described there.
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The only reason I can imagine why one might consider sending bits of BTC to random people is that you are a true believer in the concept of virtual currency and want to spread the btc's for a better btc economy... AND if you have a SHITLOAD that you don't mind spreading the wealth.
In such a case I would do that. Really.