Most of the enclosures I have in my house are used, simply because it's just too expensive to buy new tanks (in Canada, we don't have pet stores with $/gal sales, unfortunately).
However, I do ensure I'm cautious about the source (where the tank came from), I question the previous owners on what happened to their inhabitant, and regardless, I take a rigorous approach to cleaning/disinfecting. I also keep uncleaned tanks in my garage away from all reptiles until I've been able to thoroughly clean them.
And like Aimless mentioned - toss anything porous. Decorations/hides are easy to come by/make. It's not worth the risk of subjecting your new pet to possibly lethal pathogens.
Use your judgment and instincts to help decide when it works to buy used equipment. I have avoided purchasing used tanks from suspicious sources in the past - for example, people who claim their gecko "just died for no reason" after only a few years of life. Yet I've purchased many tanks from people who sold or gave away their reptile and were no longer interested in the hobby, or from people who have had reptiles pass away but at 10+ years of life. Used isn't necessarily bad - just take precautions to ensure you keep your animals safe.