One of the most important aspects about holding a nighttime garage sale is making sure it's properly lit. All your customers need to be safe from tripping over things in the dark as well as losing money and other small items if they drop them on the ground. To be sure your sale is properly lit, find one of the lighting methods listed in this article and put it to good use.
If you live somewhere near streetlights, you can take advantage of their existence by arranging your sale so that the majority of the items are in their glow. Granted, since streetlamps are usually spaced fairly far apart, you will probably have to light up the center section -- but this is the closest to free lighting you're going to find.
Floodlights are really effective for intensely lighting small amounts of space. They also suck up a lot of electricity, so if said resource is expensive where you live, you might want to reconsider. On the other hand, if you can suspend the floodlights high above the ground, they will effectively light your entire sale, and you won't need more than a few of them (unless your sale is ridiculously extensive).
Not much can be said for the strength of many porch lights, but they do at least cast a helpful glow over a general area. You may not be able to light your sale on the strength of your porch light alone, but if you add its light to another one of these methods, you could end up not needing to install or hang new lights at all (a bonus, right?).
Consider having the sale in your garage so that the garage lights can do all the work. Make sure you've got fresh bulbs in the lights and that all the areas are well-lit enough for people to navigate your little indoor space. You may need to bring a few lamps from the rest of the house and plug them in to supplement; just mark them clearly as not for sale.
Candles and tiki torches don't give off a whole lot of light, but if you use enough of them, you could effectively illuminate an area. Only do this if you have a lot of candles on hand anyway, as buying a bunch for the sole purpose of holding a garage sale is a waste of money.
We've got tons more helpful tips for you over at GarageSaleCow.com -- the nation's largest garage sale website, and a completely free resource for yard sale zealots like you and yours!