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?
Are there even tainted coins? First time that I heard about this project was also the last. There was no consensus about tainting coins and it's so easy to put them through a mixer that coinbase won't even try to identify the coins. They just don't have the time and resources to deal with it. It's the job for the feds.
That's of course unless you're dumb enough to hack another exchange and then send the coins directly to coinbase.

To answer OP's question, the 100k limit is there on purpose. If they were to say no to certain amounts they would've made the limit lower.