As with everything, there has been new technology developed. Remember when football players wore what was more of a leather head covering, than a helmet? How long a helmet will last depends on many factors, such as: was it left in a hot vehicle, garage, shed,etc. for long lengths of time--was it exposed to direct sunlight for a long period, involved in a crash? Styrofoam is not bulletproof, but will last for a while under good circumstances, but can deteriorate. So far as technology--it has advanced, whether or not you feel the need for the "best" protection is a judgement call to be made by you, but new tech is out there:
MIPS Helmet Technology: What It Is and Why You Need It | Blog - Bicycle Roots Bike Shop as well as
Bontrager Specter WaveCel helmet review | Cyclingnews. I would not buy a used helmet. And I would not buy a helmet from one of the "less than revered" overseas sites. Not saying what is "correct" or best for you, just hoping to provide some insight so you may make an informed decision.