Typically, a planet has quests for a certain level of player - this is shown when you look at the planet in the galaxy view on your space ship. The equipment you can buy from vendors will probably be on the lower end of the scale of those levels, and you'll have to hunt down where a vendor for your armor type is. You can use a website like TorHead to track down which vendors sell what on a planet, if you like.
The vendors at the fleet will muddy the mix, as they will likely have stuff for a wide level range.
Finally, it should be noted that it is most likely that you will not find the best gear by buying from a vendor. In mmos you are most likely to get the best gear as drops from instances, mobs, quest rewards, and sometimes through crafting. This is typically opposite of single player MMOs where you will hunt for that one vendor who sells the best stuff, or craft yourself the best stuff.