USPS only sells postal stamps at face value. So, if you see an ad for stamps at ridiculously low prices, they’re likely counterfeit. Using, selling, or depositing items with counterfeit postage is a fraud, a federal crime. People may be liable for criminal or civil penalties; in some cases, imprisonment can be up to five years.
A spokesperson with the United States Postal Inspector says counterfeit stamps being sold from online platforms has escalated. USPS does partner with approved vendors of stamps that are legitimate – mostly big box stores like Costco, Walgreens, CVS and grocery stores. Those stores may provide slight discounts on stamps but that is through resale agreements with the Postal Service. Any offer of 20-50% or more off the face value of stamps is not legitimate.