My preferred real estate website is Redfin. I think it's better than Zillow and Trulia, but I use those as well. All these websites are pulling from the MLS and county websites for their info and they're imperfect, but the layouts are more user-friendly and there's more info at the tip of your finger tips than just the MLS. Anyway, if you find a "deal" I would just verify it with realtor.com or whatever your local MLS website is. For example, I've found a Ben Leybovich $34,600 "pig" on Redfin...on Zillow's it's listed at $88,000, and on Trulia it's "off market". So I went to the MLS and it's there for $34,600. On Refin, there's a $25,000 house that would be great cash flow, well it was sold like 4 months ago, not listed on Zillow or Trulia. Point being, don't rely exclusively on any one of them.
As far as DOM, I think it depends on the local MLS. Some will reset the DOM if it's off the market for 30 days, others 90 days. Some track a cumulative DOM, CDOM, where if you pull it off the market and put it back on the it keeps adding days and doesn't reset till it's sold. Certainly would be beneficial to realtors to reset the DOM so the listing doesn't get stale, interest is always highest on new listings, and I'm sure it's been done plenty of time. Doubt anybody's scamming Zillow though, it's probably just pulling in bad data.