The first, and biggest thing, would be to make sure that you are following the Angel policy of whatever company's stamps you are using. I don't know all of them ... but I know for SU! you would need to stamp them with one of the "angel policy stamps" ... you can find them in the catalogue (Creatively Yours set or the personalized stamps).
Other than that, my personal ethics would say that you acknowledge (somehow) that you created, but didn't design them. But that's me.
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