It seems that a lot of people get certified, buy a lot of brand new expensive gear, and then drop out of the sport. So, you can often buy hardly-used, good quality equipment for half what it cost new.
If you DO buy used stuff, try to test things as best you can before you buy them, ask about warranty coverage, find out who will service your stuff, get as much of the paperwork (receipts, guarantees, instruction manuals, etc) as you can, get the stuff that is supposed to come with it (Computer CDs, download cables,spare parts) and have the important stuff (regs, BCs) serviced before you use it.
And if you have any questions about specific makes or models, Scubaboard is a great resource.