NYC here, original home of the stamp bag, yes, absolutely, they get ripped off, however, much to most of the time, the stamping goes on at a fairly high level, i.e. people who you would not want to fuck around with by counterfeiting their stamp. We also have quite a bit of dope that goes around in the same bags but without a stamp, which is hit-or-miss/trust-your-source stuff, although some of the best dope I've had has come from these sorts of bags (which also come in various colors which seems to be a newer trend.) The stamps get changed fairly regularly and often make amusing references to popular culture, like when American Gangster came out, there were all these retro blue colored "blue magic" stamps, the originals of which you see now, whenever a Batman movie comes out, when Obama and Trump got elected, etc.
