What others have said. Basically, don't just buy on a whim, and when you're first starting your budget, make sure you include a generous amount of PATIENCE. In reef tanks, only disasters happen quickly.
So, know what you want, and include several different alternatives that you'd be happy with if you found a good price on them. For example, if your dream skimmer is an Omega, maybe also research and find an equivalent Reef Octopus, Tunze or Skimz skimmer that would also get the job done. Don't set your heart on just one model, unless it really is the only one that is going to do the job.
Also, figure out which products tend to have stable prices, and which have come down in price over the years. Products that decrease in price - save those until your tank is closer to being ready. Products that are stable, buy them whenever you can find a good deal on them.
You can also get away with used products more easily in certain categories than others. I'd never buy an LED light used, for example, because LEDs have a limited lifespan. It's long (usually about 50,000 hours), but it's limited, and unlike with a MH or T5 fixture, you generally can't just buy a new bulb. Of course, you might decide that the lower price of a used fixture compensates you for that, but that's my preference. I'd also never buy a heater used - even new heaters can have some reliability issues, and a malfunctioning heater can absolutely wipe out your tank. The savings of going used just aren't worth it to me.
On the other hand, pumps, powerheads, tanks (in good condition), T5/MH fixtures, controllers, skimmers, reactors - all good candidates for a used purchase. Just make sure you have back-up pumps available. =)
And finally - never compromise on quality in the name of price. If you're deciding to spend less on a less desired product, make sure that it can still meet your needs. If you have to buy a replacement, it's suddenly not such a great deal, and if it leads to the loss of livestock, now you're really paying for cheaping out.