There is a comparison thread of various marketplace projects somewhere, but I'm damned if I can find it......just to be confusing, Nxt has two marketplaces, one built into Nxt core (Marketplace), and one external project (FreeMarket).
Anyway, on the native Nxt marketplace, here's a screeny of the Marketplace from today :

And here's the Wiki entry:
https://wiki.nxtcrypto.org/wiki/MarketplaceSo, yeah, thats all good to go. I've used the Marketplace myself for about 5 or 6 sales, and one purchase.
Toke, answering your questions in reverse order:
Not enough people are using it. After the great 'dark market' debate within Nxt, it kind of got shoved under the carpet.
So badly shoved under the carpet that it's not even mentioned in the Nxt Wikipedia entry:
https://en.wikipedia.org/wiki/NxtOoops....
Escrow is pretty basic: if you make a purchase, your funds are held until the seller confirms delivery/despatch. If the seller fails to confirm, you get refunded.
As all sellers are linked to Nxt accounts, the reputation of a bad seller will quickly get burned if she/he failed to deliver, and the (planned) reputation system would add further pressure on sellers to build up and maintain a solid reputation.
OpenBazaar is more like Nxt
FreeMarket, which is a third party marketplace app (using Nxt messaging, strangely).
Here's a couple of FreeMarket info links:
http://nxtfreemarket.com/https://nxtforum.org/nxtventures/freemarket-official-thread/Lite version:
https://freemarketlite.cc/FreeMarket development has been slow lately, but 1.5 should be turning up soon.
Any specific questions about FreeMarket, I'll kick over to the FM devs....