Might work differently from market to market so let's say for Evo.
example
Lists at 20$ - Evo then creates and updates the listing to match current BTC value.
Lists at xxxbtc - Vendor then reposts(or edit?) listing manually to account for btc fluctuation
Which one is it?
They list it in USD and it automatically gets converted into BTC. Vendors don't have to manually update prices.
If you have any further questions I reccommend you check out https://reddit.com/r/DarkNetMarketsNoobs