Is starbound a single world with different characters

Since the world seed for each server or local game is the same thing, and the universe (as a whole) is the same for each planet, it would be possible.

To do this, you'll need to know what X/Y location you're on for the galaxy map. Then, find what planet you're on. When you have found that, place a chest on that planet and put your stuff into it. Then, log onto the new character. With the new character, fly to that location and then grab the stuff.

The map generates as you explore it by rules set by the game (same locations always look the same in same version of the game). The whole planet is not generated at once, but in smaller chunks. This can be seen if the server can't keep up with your exploration.

If you create a new character, that same planet, which previously had naked merchants, will continue to have naked merchants. You can fix it by regenerating your universe by deleting the universe folder.