At my local supermarket they are selling peppermint seeds. How is it possible? I thought peppermint was a sterile hybrid, and only reproduced through stolon.
TL;DR: They're seeds that will grow up to be a peppermint, not seeds that were made by a peppermint.
Yes, peppermint is a hybrid (specifically an F1 hybrid) of two other species of mint, spearmint and watermint. Hybrids between species are generally sterile, but since the parent species are fertile, one can pollinate the other to make the peppermint seeds that you're seeing. The hybrid plants share characteristics from the two parent species.
(Another, much more remote possibility: most F1 hybrids are sterile, but they can sometimes be fertile; their children are called F2 hybrids, and their characteristics can vary across the entire spectrum between the two original parent species. It's possible that your supermarket happened upon a source of fertile peppermint.)