Most collectors know that it's illegal to counterfeit US postage stamps. In an attempt to defraud the USPS, some stamp issues have been counterfeited and sold. These counterfeits look to rip off the USPS. The US postal inspectors will try to hunt you down.
There is a second kind of counterfeit out there aimed to rip off the collector. These are the ones you need to be aware of. And the reason I mention it is because as I write this, I've seen some items on eBay recently that just look ripe for being fakes. They look too good to be true. These items are not accountable paper like postage stamps and there is no postal law against counterfeiting them.
One area in particular that is a problem is local stamps. These are listed in the Scott catalog with the "L" prefix. Most of these were created in the mid 1800s. In many cases, the local stamps were crudely printed by the local print shop under contract with the local postmaster. The designs are simple. When they were created, the local postmaster didn't worry about counterfeits.
Turn the clock ahead 150 years and it's a different story. Thanks to computers and other technological advances, it's now very easy to create counterfeits or reproductions of these items. And then sell them on places like eBay where uneducated buyers think they are getting the genuine item at a bargain price.
Local stamps, carriers, postmaster provisionals, post office seals, test stamps, and several other back of the book categories of stamps are not protected by postal law against counterfeiting. The designs are simple and easy to fake. If you're interested in this type of material, the best advice I can give you is to buy it from reputable dealers who have a track record in handling this kind of material. Or seek expert certificates for items you buy. Please, there is no free lunch. Caveat emptor!