These are just four of the dresses that come up today if you search for 1920s vintage dresses on eBay. What do you think?
#1 "Original 1920s ART DECO
BEADED FLAPPER DRESS"
#3 "Art Deco Voided Velvet
Beaded Drop Waist Dress"
Solely on the basis of these pictures, I would say the first one on the left is the only one pretty surely 1920's, the 3rd one possibly, but without closer inspection, could be a repro from any decade since.
#2 "DECO Vtg 20s Jewels
Flapper Sheer Dress"
#4 "c1920 Flapper Dress
Thousands Of Sequins"
Dress #2 is stylistically right smack in the 1930s, but I would bet my eye teeth it is a modern repro — I would also suggest it is lingerie rather than an evening dress. You could easily wear it as an evening dress, if you either a) found the perfect slip for underneath or b) got a lining made for it. Dress #4 I would suggest is made in India and dates anywhere from the late 1960s to present day.
What's my point? Firstly, don't believe that something is 1920s (or any other decade) just because a seller thinks it is. Many of the sellers on eBay are laypeople who are just doing their best trying to guess what the items they are selling actually are — most of the errors are usually in good faith, with no intent to deceive. Secondly, any of these dresses worn with a slip (black, silver, gunmetal, white, &c.) would make a fabulous dress at a great price and guarantee you a pretty unique outfit. If you fall in love with something, who cares what its actual provenance — if you get what you think is a great deal on it and you love it, that's all that matters. But, if you're on the market for an actual "antique", always keep in mind the motto, Caveat emptor, buyer beware.