Do Guards/NPCs Respawn

I've found the best way to make money is pick up ingredients in the wild, make potions and sell them.

Easy to find

Wild ingredients are easy to find. Or you can buy ingredients from many vendors. Avoid buying expensive ingredients like void salts, fire salts, daedra hearts etc. The return isn't worth the cost.

They're light

Most ingredients weigh little. If you make a lot of potions and can't sell them all, the potions also don't weigh a lot - 100 potions weigh about the same as 1 piece of heavy armor. Their value/weight ratio is high.

They sell for a lot

I regularly sell a 0.1 weight potion for $200. There are other items with higher value/weight ratios, but I can quickly make 10 of these with ingredients that aren't hard to find. This works best for invisibility, because you can sell it for squillions. I've made around 20k mostly from potions.

I have seen potions with values in the thousands!

It's good for your alchemy skill

It'll also help your alchemy skill, and as your alchemy skill gets higher, you'll make more valuable potions.

You'll always have potions

There's nothing worse than getting into a battle without the right potions. Having a few spare potions for health and magika on hand will help you a lot!

Win win win!

Generally, they'll attack you.

This can make the assassination quite a bit easier, since you are technically acting in "self defense", and you will not be given a bounty for killing the target. You will also not draw aggro from otherwise neutral NPCs in the area, like you would if they noticed you attacking the target in the first place.