Its pretty safe. Many online retailers specialize in just saltwater fish hobby, and they are not big box store types whose employees don't care much about return customers. Also, the variety is much much wider. With your small tank, however, you are limited in what you can have and how many fish. Your tank size would be considered a nano and you will be limited to a couple of small fish. If you have a decent fish store nearby, I'd start off with that first. Don't let sales-people talk you into getting fish that are unsuitable for that size tank. Do your research first before diving in.
I'm going to move your topic to the New Hobbyist Forum, since this particular forum is for off-topics.