There's probably less you can do in practice than theory. You can copyright the stuff but the legal costs of enforcing may not be practical for a smaller operator.
Established yourself as the clear originator of the "original authentic" product may slightly help discourage people buying copies.
In some cases sending a Cease and Desist Notice to a site selling your stuff, with the threat of legal action helps discourage a few pirates.
Perhaps a US Digital Millennium Copyright Act filing can be made against identified pirates.
http://en.wikipedia.org/wiki/Digital_Mil...
Copyrights can be worked around by making changes to the copied material, requiring at least a court case to determine if it is indeed derivative or original work.
Some selling from an anonymous site may require a complaint lodged with their host, registrar or Payment Processing service to effect them.
Different software protection schemes have been tried in the past, they end up inconvenience users more than inhibiting pirates who seem to almost always manage to crack protection systems.
Find out where they are reselling it and offer your product at the same place.
Make your product better so people will want to keep it. Why would you stop them from reselling something they purchased? That would not be fair, unless you agree to refund them what they paid if they're not satisfied with the product.
RAise your prices so there is no profit in reselling your product.
raise your price now
I am making a product (digital product) for my store. Once I make it, people just buy it and resell it to make money. How do I stop this? When people resell my product it hurts my business.
