ha! Soon we'll be hearing about stamps counterfeiting...
They do seem rather vulnerable to me. What is the hard part exactly?
Stamp counterfeiting, at least for U.S. Postage within America, can be prosecuted under similar or the same laws as counterfeiting currency. It is a dangerous game so far trying to play that game, and puts you directly under the jurisdiction of the U.S. Secret Service.
Stamps are also made with security considerations, including things that don't normally get publicized as well, so I don't think it is nearly as easy as you would think. If something seems a little off, it would be noticed. Also, most people are not used to buying stamps from ordinary people or from a random stranger like at a flea market. If they aren't sold at a post office, they are usually sold at a brick and mortar retail store that has a reputation to keep up, where they won't be selling stamps except from a distribution chain that ends up coming from the USPS.
If you are selling to the philatelic community, expect even more scrutiny than had you been selling the stamps to a secret service agent directly. Good luck with that one, where a counterfeit stamp would be rooted out so quick that it would make your head spin... at least for some schmuck that doesn't care about stamps. You might get away with it, but I think you would find it easier to simply counterfeit a FRN if that was your goal. Or more significantly, simply buy a bunch of GPUs and start mining bitcoins instead... you'll be money ahead by doing that.
What is being done for postal counterfeiting is mainly with "exotic foreign countries" of some sort or another and trying to pass them off as the genuine thing outside of that country. Again, this is a pointless exercise, and you would be better advised to deal with a micronation like the Hutt River Province or Sealand and simply make it "official postage" if you want to go through that sort of effort. That sort of is the point where the cost of making the postage is presumed to be nearly the same as its face value, except when made in large volume. Perhaps a good home printer can make some stamps that you might pass off to somebody else, but is it worth the effort?