Ok, what's the best way of defeating the sandpeople? I know it was suggested to buy the protocol droid, but that was for 2500 credits, which I don't have... So, another question would be, how do you make the credits for it?

sell carlo nobs or whatever his name is armor and u will earn enough money

Swoop Bike racing is one way to make some money. Sell the racing bonds for credits. This will give you plenty of cash for the droid.

I've managed to make a lot of money just selling the dozens of blasters and what not I've accumulated throughout the game.

If you do all three tiers of the racing, you can get, what, 7000 credits? A whole bunch. Plenty for the droid, and some left over for the ladies in the Cantina.

You want the droid. He's a sadistic little killing machine with a couple of blasters, and his dialog is some of the funniest in the game. He's fun if your light-side, and an absolute blast if you're going evil.
Better yet, if you have decent computer use on your main character, you can get a chance to 'repair' him, and make him very powerful. Besides, as you'll find out later, he actually is an assassian bot that used to be owned by your character.