Well, I think not many people expect somebody to fake a rather common coin and "deface" it afterwards with fake chops that even detract more of the value.
They are prepared to look very cautiously at rarer coins, know that there are counterfeiters, have heard of things like "altered mintmarks", maybe even seen them, and would never buy a 1,000 dollar item that's being offered at 100.
But a common bust dollar, circulated and even with some of these illegible Chinese marks, come on, who on earth would counterfeit them?
And bang, there we go...