They might not care about the size of the transactions, but they definitely care about the origin of the funds.
E.g. if you want to sell 40 BTC they won´t have a problem if you acquired them legally and can prove it.
Does Coinbase itself care? All I know is that they're required to flag and report transactions of certain amounts, and then it'll be the feds' job to determine the origin.
Then again, Coinbase probably wouldn't want to buy tainted coins. Can they refuse sell orders if they somehow determine that the source is unclean?