In person: Look the garment over very thoroughly . Hold it up to light to see if there are any holes. If the shop is dark, take it to a window to check the armpits and shoulders. The two places to always check are the shoulders for fading and the armpits for stains. These are the most likely areas for problems. If the shop is both dark and has no windows then take the item to the door or reconsider shopping there. Serously. Bad lighting hides a multitude of sins and unethical sellers know it.
Online: Actually READ the description. Don’t just scan it. Ask questions. If there aren’t a lot of photos, ask for more. Don’t Assume. Flaws should be mentioned specifically. Check the Return policy - even a No Returns for Size Policy should have a caveat that the garment can be returned if it is Not As Described. Even honest sellers miss things. Phrases that should set off an alarm:
It’s In Good Shape for It’s Age
An Experienced Seamstress Should be Able to Fix This
There are a lot of systems. Mint is Mint, i.e. Perfect to the point of Unworn. If an item is ‘Mint except for X, then it’s not Mint. Excellent is not Mint. It has been worn, or can have flaws so minor as to be unnoticeable. All the others, from Very Good to Good to Fair to Poor are very flexible these days depending on the seller. If the seller has a Condition Rating Chart be sure to read it. And be very careful about asking questions on anything that is rated less than excellent. If they don’t rate the condition - tread very carefully.
Special Warning Division: 1930s and 40s crepe dresses were dyed with fugitive dyes in many color ranges. The red, blue and purple are the worst for fading in streaks. The black tends to get rusty red. Be especially careful looking these over in good light. Net dresses need very careful inspection - there are usually holes in the full skirts. ‘Loose threads’ on a 1920s beaded dress most likely means missing beads. Theses can be quite difficult to replace.
Once you have handled a lot of pre 1980s vintage, you will be able to spot a reproduction easily. In addition to the style and cut if the garment, the construction techniques, fabric and trims will tell you. What is now getting harder is that designers are copying the period looks even more wholesale than in the past. In the 1990s, I believe Givenchy did a new line that even used his 1960s patterns. The stores are full of 1970s maxi dress knock offs now. Knowing your fabrics, fasteners and labels will help.
There are a lot of 1920s - 1950s reproductions being made. And sold as just that - Repros. Nothing wrong about it at all and there are some very nicely done pieces out there. But these can get moved into the secondary market as Vintage. Be familiar with the labels and latest lines. That will help you avoid paying top dollar for a very nice reproduction. And use caution. If you find a garment in perfect condition that has all the earmarks of a period - i.e. a 1950s ‘bombshell’ style dress in a pink and black Atomic print with coordinating crinoline, and labeled Betty's Boop! beware. When a garment hits on all the cliches, it could likely be repro.
This is where knowing your dating and your details will save you.
Favorite Stories: 1) Attending an auction of Vintage Clothing that was described as “In Good Condition for Its Age“, which translated as a warehouse of dry rot, shattering and filth. It was a 10 hour drive round trip. 2) A Victorian reproduction costume I designed myself turned up in an Antique Mall that was labeled and priced as the real thing!