What are the most efficient uses for the first 100 gold that you get?

You can get a pirate realm key!

And you might be able to dye yourself a few times.

And that, is it.

I'd say don't bother.

Since you get most of the good stuff by looting there is nothing useful you could really buy for 100 gold.

Lately, if you had 100 gold, I would recommend you buy the backpack. It gives you another eight slots to hold items, which is pretty useful.