Rent vs. Buy?

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Besides the "personal" sorts of gear like wet suit, mask, fins, I'm looking for some opinions on the age-old question:

Should I rent or should I buy?  (or)  Buy used or buy new?

My own situation is that I finished my open water cert this spring after putting it off for 10 years because at the time, paying ~$300 for the class by itself was a costly venture.  Since then, I got hooked while diving in the Caribbean on my honeymoon.  Now I'm quite sure that I want to progress in the sport.  I scheduled my advanced class for later this month and I have a feeling I want to continue through the rescue diver and onto a specialty in photography/videography.

Currently, I don't have to worry about renting gear because my instructor includes the use of gear with the class.  But that won't last forever, obviously.

However, most of the gear for sale on Craig's list, etc. seems to be:

"tech" gear (which I don't need yet)
"garage sale" gear (relics from the 70's that you probably wouldn't feel safe using anyway)
"didn't dive as much as I thought I would" gear (which costs 10% less than buying new)

Have you had any luck buying used and trading up?  Or is it best to start just buying some things completely new, one-by-one?

Extra Credit:  How can I justify this to my wife?  :