Here are some tips that will help you when it comes time to buy fireworks.

Buy from a reliable source

Most of the time, locating a licensed, reliable firework outlet is no problem.  These stands are usually marked by flags and signs indicating the manufacturer/importer's name.  Make sure the fireworks that you are buying have labels that indicate a manufacturer's name and warning message.  These fireworks were made and tested in factories with your safety in mind.

Buy as early as you can

After running a firework stand for the past few years, I've noticed that most people don't buy their fireworks until July 3rd or 4th.  This is a bad idea not only because it creates huge crowds at firework stands, but by the time you get there, most of the good items are gone - much like doing your holiday shopping one week before Christmas.  This is especially true with novelties - they are one of the first things to sell out, and people who come to buy them at the last minute often can't.  Also, getting their before anyone else allows you to ask the stand operator any questions you have about the products.  So if you want to be sure that you'll get the items you want, get your fireworks before the rush.

Buy assortments

Assortments are the best deals you can get when shopping for fireworks.  Sometimes you can get up to twice as much fireworks for the same amount of money you'd spend on single items.  So if you plan on getting more than a dozen or so fountains/repeaters, or you want to get a wide range of stuff for a backyard display, or just want to get a bunch things to entertain little kids, consider getting an assortment.

Check for damage

It's a good idea to examine fireworks prior to purchase before buying them.  Here's how to do a quick inspection:
    -Make sure there's no water damage: this isn't a big issue with items in plastic packages, but anything with bare tubes or paper should be checked to make sure that they haven't absorbed water (indicated by swollen tubes or bleeding ink in the labels).  If it has, there's a good bet that the firework won't ignite.
    -Look for loose powder: hold the device over your hand and gently shake it to make sure nothing falls out.  Small amounts of brown powder (clay) are ok, but if you see any gray/black/white powders coming out, don't buy the device.  Loose powder could cause the device to function improperly or not at all.

If you find anything wrong with a firework, just ask for another one instead.  The stand operator himself doesn't actually have to pay for the damaged device - they usually are reimbursed by the shipping company or the wholesaler.  

Ask about product guarantees/dud returns

Some firework stands have a product guarantee and will refund or replace any fireworks that don't function properly (duds).  However, most places that have full guarantees won't often advertise it.  Ask your local stand operator if they have such a policy. 

Watch out for bogus deals

Unfortunately, not all "Buy 1 Get 1 Free" type deals are designed to save you money.  Similar to regular stores, some "Buy 1 Get 1" deals are rigged simply by doubling the price of one item, giving the buyer the impression that they're getting twice their money's worth.  Trying to differentiate between true bargains and rip-offs in the world of fireworks can be difficult - before you decide to take advantage of such an offer, look around for the prices of similar items.  Get a good feel of what the average price for a certain type of item is, so you can figure out if the "Buy 1 Get 1" deal is really worth it.  For example, say you come across a $20 "Buy 1 Get 1" deal for a certain large fountain.  This may sound good, but if you look around, you might find that a very similar fountain is sold for $7 apiece.  In that case, you'd probably be paying extra for the "Buy 1 Get 1" deal.