Most of these scammers live outside of North America, probably right next to that delightful Nigerian prince who just discovered that I am related to him and wants to give me bushels of money, right after I give him my social security number as ID. (cue the sarcasm music)
The post marks are usually in the USA because there are remailers who will mail hundreds of pre-addressed, pre-stamped letters for around $10 total. So that letter you got from Friendly Small Town, USA probably originated in ScamCity, Somewhere Else. That's why they operate with impunity. They are outside of US jurisdictions, they have very little invested per person duped, and they are absolutely heartless and despicable.