As far as the little I know about 'costume jewelry' ... it's got a long history , in many forms ... in the 1700's any item with glass or 'paste' 'jewels' was considered to be of this type (sometimes copies were worn to avoid the theft of the real deal) , and many had basic construction with precious metals .
Around the mid-ish 1800's , rolled gold (a thin layer of gold , 'fixed' to a base metal or low content gold/silv/plat) became available at a much lower cost than the 'real deal' , and were rather popular for some .
Fast forward to the late 1920's in Europe where they had a boom in sales & production of 'rhinestone' jewelry , which entered the mainstream market in the U.S. in the 1940's .... some very low-quality items along with well made ones too !
Costume/fashion jewelry was quite popular with the 'flappers' of the 1920's & 1930's (back when money was tighter than it is now) !
Costume jewelry items , for stage performers was also known as 'stage jewelry' ... nowadays they/it is know as 'props' .
Maybe next time someone tries to correct you about the term 'costume jewelry' of the type you mentioned you were selling , debodun , you might consider giving them a wink & mention Halloween !
I personally think of any item of jewelry that is not 100% "the REAL deal" (stones &/or metal/artisan) as 'costume jewelry' (with many exceptions for antique & ancient jewelry) ! That's just my form of 'old-school' thinkin' ...
& by the way , nowadays even good auction houses will consider and advertise 1930's stuff as antique !
Great question & list of opinions on this one !