If you're a pirate, then you can attempt to pirate the person (assuming they're not a member of a friendly or allied faction). Demand the cargo, etc. If you can't demand cargo, then no, you can't attack them... If there was a good enough reason for you to attack them in the first place, then you probably shouldn't have been selling stuff to them.
Most people do trading near a base so that people can dock if someone tries to gank them for the loot. An arms dealer who tries to kill his clients isn't going to be doing much business in the future, and someone who tries to deal with an arms dealer and then kill him is unlikely to find anyone willing to sell him stuff in the future. Bad rep is a bad thing.
