If you're one of those
peel off that stamp with no apparent postmark so it can be re-used people, it will eventually get you in
serious trouble.
Some of the automated machines use a non-visible postmark ink for the stamps. During the pre-sort, some of the mail gets run through the sorter, which separates the mail by zip and looks for cancelled stamps. Not all of the mail gets run through the process, but in major metropolitan areas you can rest assured they pre-sort. Sometimes they sort out the fraudulent stamps and send it to the Postal Inspector, who follows up. They log where it came from and the destination. When they have enough evidence of fraud, or if they wish to make an example of someone, they go have a nice chat, mentioning things like fraud, felony, jailtime and other enjoyable catchphrases.
You take your chances if you do it often. I know of one person in an insurance office who peeled off the stamps and used them for personal use. When the government visited the office, he was fired and had to pay a fine (which is hard to do when you're now jobless). It isn't worth the 34 cents (in the US) to get screwed.