We've talked about this before but here's my contribution. Everyone should bear in mind that these are my opinions although many militaria collectors in Germany use similar distinctions. IMHO, the militaria collectors haven't adopted standardized terms (similar to that used by medal collectors) which leads to misunderstandings. The questions DerAdler pose are ethically interesting. (Where DerAdler and I historically differ on this topic is mainly the terminology rather than the spirit of the answer.)
1. "If a dealer/collector takes a plain German helmet and adds authentic SS decals, does that make it an authentic SS helmet worthy of $2000+?": It'd be fairly hard for me to believe that contemporary SS decals could still be applied and suspect that replacement decals were used. With the former, the helmet and it's markings would be contemporary but not as issued (hence a lower price) while the later would be in the re-enactor's class (and not worth the price of a contemporary or as issued helmet). A major concern to me would be the description of the item. Decal application can be analyzed and sometimes dated but the experience level for this is quite high and is not always possible and is even more difficult if contemporary items are used.
Consider this example: You have an authentic/official attributed as issued Blue Max with Oak Leaves worn in WWII by a member of the German Staff. The ribbon is missing. (1) Do you replace it with a current length of ribbon? or (2) Do you replace it with a contemporary length of ribbon that cost $200? or (3) Do not replace the ribbon and just leave the Order bare of ribbon in your display or re-sell? or (4) Keep it and work on a upgrade to a similar item which has never had any part of it replaced? (Hint: In medal collecting there is a standard answer for this example.)
2. "If a dealer/collector adds an original Gross Deutschland cufftitle to an otherwise ordinary German infantryman's uniform does that make it an authentic Grossdeutschland uniform?": It doesn't make it an contemporary authentic as issued Großdeutschland uniform. This particular example is a little easier because the thread used for sewing can usually be dated fairly well and a quick determination of whether this is totally contemporary or not. With all original items used, it could be considered authentic but not as issued or not as worn.
Consider this example: You have a totally authentic, as issued contemporary Großdeutschland uniform which is heavilly documented, named, and attributed beyond question. Unfortunately, the Iron Cross 2nd Class ribbon is in frays but luckily you have an original ribbon piece laying around and replace it. You identify this in your description. Does this change the uniform? (To me it would and render the uniform totally uninteresting, just for this one minute detail.)
3. "And if they identify in the item description that the item is original but is put together using authentic items, should he be penalized for selling fakes?": In this particular case it wouldn't be a fake since the alterations with original items was clearly identified. However, it also would not be contemporary (which is where the presumed value is established) - that is what you're driving at. By defination, this couldn't be a fake since all alterations were clearly identified. Determination of these alterations would be fairly easy by looking at the threads and ares where the alteration was made.
