All your images on rgbstock.com are copyrighted to you. You don't have to agree to use outside the licence.
I received two requests to sell two of my images today, and there was a sad story attached. No other information was given.
If you don't care if someone makes money selling your images, then fine. But you do not have to agree, and since there are lots of public domain images out there, it is most likely to be a scam.