My limited experience trying this suggests that, indeed, lots of research is needed in order to know what is likely to sell. The market is overflowing with secondhand formal dresses, and even lots of brand new ones are available cheap through resellers of overstock.
Here's my limited experience: I tried to sell a wedding dress whose original retail price was $3000 that I'd bought for $100. No luck. The bridal resale sites wouldn't take it without an original bill of sale from a salon to prove it was less than 2 years old. Repeated listings on EBay and craigslist didn't even produce nibbles.
I don't mean to discourage you--just to emphasize that you're right about needing to do research.