I have been through the FAQ and the wiki back to front and I can't see any clear explanation of what a multi-sig marketplace is and how it works. I have tried searching and can only find posts about such and such a marketplace now supporting multi-sig, but nothing about what it actually is. I am guessing it is some sort of way of decentralising the escrow to stop marketplaces stealing all of our bitcoins but that's as much as I can gather...
3 keys. You've got one, the market has one and the vendor has one. 2 parties have to agree to release funds.