Maybe we could learn ourselves...
Nothing is worse than buying a coin online only to find out that it has been cleaned, and at that point, there is almost nothing you can do about it.
So... what can be done you ask? (You DID ask, right?) Learn to tell yourself, wether a coin has been cleaned or not. There are a number of books on the market that go into great detail about coins that have been cleaned. Buy or borrow a book or three and learn for yourself about cleaned coins.
With knowledge in hand you are ready for battle. When you spot a coin on the net that has piqued your interest, ask the seller for a detailed picture, a close up scan. Looks for those signs of cleaning. Short of that, dont buy raw coins online, or if you simply must buy, then buy at a discount, counting on the coin having been cleaned. With this knowledge of cleaned coins though, you are in a much better position to go to a coin dealer, or coin club and be a better judge of the coins that interest you.
When we as collectors rely solely upon the word of another, we are certainly setting ourselves up for disappointment.
I have a great little book, titled "Coin Chemistry (including preservation and cleaning)", by Weimar White. Bought it on ebay for a song. Awesome book.
This coin was in a ANACS slab as MS68, crossed over and got promoted.