If I understand correctly the mechanism, my public key may be used by other people in order to mess up the traces, and that's what grants Monero and other ring-signature based coins its untraceable nature.

So, is there a way to know which transactions have already used my public key?

I ask because I'm trying to do this https://www.reddit.com/r/Monero/comments/3ctjed/friendly_reminder_to_bitcoin_users_that_seek

I'm planning on having two Monero wallets. Buy Monero with ShapeShift into wallet A, transfer with high mixin (10?) to wallet B, and sell this Monero against Bitcoin, to recover coins not linked to my original address. Perhaps swap back and forth between A and B a few times before selling.

If I make the transfer from A to B right after receiving money on A, it would be a bit too obvious which is the public key actually been used, regardless of how high my mixin is. So my plan is to wait a while, and don't transfer the exact amount. Ideally, I would transfer after my public key has already been used by somebody else.

So, is there any service that shows this to me?

Thank you!