The more popular a site like this is the better the reputation systems work.
Durability against shutdown, lttle/no downtime + fast loading speeds would help increase transactions and therefore opportunities to build reputation, or scam, cutting the wheat from the chaff.
At the moment you hardly get any ordinary businesses on these kinds of market places but if one was set up that actually functioned as smoothly as ebay but without the fees, then it could bring in far more users.
e.g. online advertising. Having advertisers + content creators matched, like Google adsense or Izea https://www.youtube.com/watch?v=DyitfcJhRXA
, but with much lower fees. If the currency used had to be bitUSD that could being enough income to be allow virtually no fees. The only fee is - risk using bitUSD. There could be an online advertising section, someone could put up advertising space for sale on their blog. Advertisers buy it.
Another market would be the buying/selling of websites, like flippa, but on bitzaaar.
The site just needs to be reliable enough for people to trust it with other types of goods.