My thoughts so far:
*easy to use Monero multisig transactions.
*internal messaging
*forced GPG messaging
*two factor authentication
*good product description
*small, light weight website so it loads quickly over Tor/other anonymity networks
*no centralized wallet to stop exit scams
*robust rating system that protects buyer
*fail-safe: if it gets taken down, no one should lose money or have potentially sensitive information leaked.
*affiliate program - this will help sellers sell more.
What else?
NOT BEING RAIDED BY LE
PURE DRUGS AND LEGIT VENDORS
Oh I forgot