you've unfortunately been the victim of such a payment scam, you should contact your bank immediately if you've given out your details, and you can report the incident to your local police station, to Action Fraud (for England, Wales, and Northern Ireland) or Police Scotland (for Scotland...
Some of the websites take payment and then send out counterfeit stamps. There are other websites which are capturing customer details but not sending any stamps. Below are some examples of scam websites. Click on each image to see a larger version. ...