Please remember that these people are experts at check washing, they use chemicals to wask the ink from the checks where they need to fill in the blanks to reissue them.
Checks and money orders a like are either given to them through purchases made by others online, or they get them through other means such as mail interception etc.
The check or money order could very well check out on a pysical inspection to be authentic but what the recipient does not realize is that it has already been cashed by someone else and has now been washed by the scammers and reissued to someone else. OR it was issued for a much smaller amount and the dollar amount altered.
Very rarely are they washed checks, most are simple reproduction of a single check. On moneygrams the heat sensitive feature never works and the postal money orders are all in numerical order.
Simple education on what to look for is the key.
BTW I am training people in Houston today to id counterfeit checks