You may want to add another type of seller to this list?
How about the seller that sells improperly cleaned coins without mention the coin has been cleaned? This happens quite often on eBay with the raw coins, particularly with copper/bronze coins. I know I have been burned a couple of times myself.
I've learned to ask before I buy. At least that way I have the dealer's written word if it ends up having been cleaned. Maybe a refund comes a little easier that way.
For those that don't know anything about coins or those just starting out, it very easy for the sellers to take advantage in these cases, especially with raw coins.
I agree! Stand up and SHOUT about it...