A large portion of my kit is used. Mask, fins, snorkel, boots and weight belt were new. Current BC is sorta new (was returned to factory and refurbed, sold for about 70% of new with full warranty). First BC, wetsuit and Regs were former rentals from my LDS. Weights and all my tanks (5) were used (although 2 of them were reportedly only dove twice, former owner killed in an MVA a month after he purchased and they languished an a basement for 2 years, CAtalina AL80's, not even a scratch on em). I've bought 2 computers (both Oceanic round style), both less than half new retail. The second one was advertised as the computer module and arrived in a console with spg, that I like better than my current unit (more compact), just have to swap and transfer my compass. Also just bought a 6.5mil semi dry on Ebay, for about 1/3 of the asking price of my local LDS. Can't see where it's ever been used and a perect fit.
Rule number one, is don't spend more than you can afford to throw away. On Ebay, yes be careful, but longer term sellers value their "feedback" and don't mis-represent their product. The only item, I ever bought that I didn't think was fairly described, was immediately accepted back and a full refund received.
Kent