As far as I know most stamps should only be used for personal use, or gifts, not for making stuff to sell. But there are quite a lot of what are called 'Angel' stamps, which have copyright free use. Then there are also many that allow a limited amount of use for selling, so long as it isn't on a large commercial scale. If you know the manufacturer of the stamps you want to use, you can probably look them up and find out their policies.
Look up Angel stamps and see what you can find. The alternative is to make your own!
I am also fairly sure a lot of people aren't aware of it possibly being an issue, or if they are, they just ignore it and carry on anyway and sell their stuff. If I use stamps for anything I sell, I try and make sure it's ok for what I want to do with them.
Shelley