Whether it's for re-enactment, teaching, stage props/costuming, or a cool piece of decoration -- people who are looking to buy a reasonably-accurate replica of a Roman-era helmet don't have to commission them for several hundred dollars and wait several months for them to be handcrafted.  Over the past decade or so, several companies have started mass-producing them, so they are a popular off-the-shelf item at many online armouries, and on eBay. 

However, there is a lot of junk out there that's advertised as authentic reproductions -- or worse, artificially aged and passed off as genuine historical artifacts.  So it's a good idea to do some homework before buying.  Matthew Amt of Legio XX (their website is a great resource, and many other Roman re-enactment groups have adopted those guidelines) notes that it's possible to get an historically-accurate helmet for not much more than you'd pay for something that isn't at all authentic.  One practical benefit of getting one of the "good" helmets is that it will have a much higher resale value than a cheap, junky one -- besides the fact that if you're onstage or in a classroom, the chances are pretty high that some history buff in the audience will know if you've got the wrong stuff, and call you on it!

There are a bunch of well-illustrated books, covering the period from the early Roman Republic to the Late Roman era (e.g. the historical King Arthur) that can help show you what to look for.  Peter Connolly (Greece and Rome at War), or Bishop and Coulston (Roman Military Equipment), are widely available in many college and public libraries.  H.R. Robinson (Armour of Imperial Rome) has a lot of photos of helmets, along with re-created examples.  There are also several Osprey guidebooks that deal with Roman legions, which can be helpful for suggesting armour, weapons, etc. -- they cover all the helmet types from quite early on (Attic, Montefortino) through to Julius Caesar (Coolus), then Claudius, Nero, and beyond (Imperial Gallic, Imperial Italic). 

Some unscrupulous vendors have begun sprinkling technical terms through their listings, claiming that a particular helmet is Gallic or Italic (sometimes both, which is pushing it), in order to appear more authoritative -- be warned!

A few hints for spotting poor replicas:  the Romans did not use stainless steel or aluminum (this goes for swords as well!).  Neither they nor the ancient Greeks made their armour by casting brass or bronze into a mould -- a thick, blocky helmet that weighs a dozen pounds is probably a recent tourist souvenir.  The classic type of Roman helmet with a peaked plate across the forehead, found at Ottawa Senators hockey games or in Asterix cartoons, is actually not typical of the imperial legionary helmets that have been unearthed -- check out a reference book, or the website of a Roman group like the Ermine Street Guard or Legio XX.  A neckguard that comes to a point at the back is also a bad sign!  Crests were made of horsehair or feathers, stuck into curved blocks of wood (not metal), and they were removable, rather than being built directly into the helmet. The metal bar riveted across the front of many Roman helmets isn't meant to flip up and down like a visor (some sellers have photographed it out of position).  Also, the curvy eyebrow-like decorations on Gallic-type helmets should be embossed right into the helmet, not cut out (especially not out of brass sheeting) and soldered on.

It would be a pity to pay a hundred bucks for a poorly-made helmet when there are much more beautiful and accurate replicas going for less than that, if you know what to look for.  Even at fifty bucks it's not a bargain if you're going to be stuck with it -- or worse, end up lying to a novice buyer in order to get rid of it!  (You don't want to be that kind of shyster, do you?)  Remember -- one of the old "Roman Trooper" helmets with the pointy neckguard might have sold for a couple of hundred dollars, less than a decade ago -- but since these poor-quality knockoffs have flooded the market, the unfortunate buyer would probably not get back a quarter of the original price upon resale.  Increasingly, these offerings on eBay are failing to get any bidders -- so even if you ARE that kind of shyster, you'd be out of luck. 

If you are concerned about having to clean and rustproof a non-stainless steel helmet, consider a bronze or brass one.  You might have to rub it occasionally with brass polish, but it will hold up well in storage, and it's historically accurate -- a number of different brass/bronze examples have been found, from most Coolus helmets to the Italic C and Gallic I, the spectacular Italic H (for the 2nd century AD), and auxiliary and cavalry types as well.  By the way, if you pick up a secondhand helmet that's been badly neglected, rust can be scoured off with various grades of sanding pads (available at hardware or woodworking stores) and even the green scrubbing pads used in the kitchen.  A good helmet will still be usable (and even acquire some character) after it's been knocked about, but once the plating wears off, and the ornaments come loose, a junky one will just be ready for the scrap heap.   

Besides the "What Not to Buy" alert page, the Legio XX website has a lot of helpful tips about how to modify various off-the-shelf Deepeeka helmets so they look more authentic.  This can actually increase the resale value, to informed re-enactors and historians. 

Lisa (Legio XXX)