Selling items that are related to a copyrighted creation- like Star Trek and Star Wars-without permission, is illegal because you're cutting into the rights of the creators to make money from their creations.
 Luckily in the case of Star Trek, Paramount - and now, hopefully, CBS Studios- have a history of accepting the fact that Fan Fiction like the fan films and my own Nova Trek project- are going to be made. As long as these projects are offered for free and no profit is made from them, the studios more or less look at them in the same vein as free advertising for the "Offical" product and turn a more or less blind eye to them.
 But if those artists who are actually selling projects related to copywrited materials are ever reported, They can find themselves in alot of legal trouble.