A number of my friends have used Swappa with success. They seem to have a competent return policy and most sellers include a checklist of quality steps they look for. All listings have a top level quality grade you can filter by.
They do seem to be a little higher price than what you can find on eBay or Craigslist, but require less "digging" to find a good phone.
In the US Bestbuy sells their returns as open box and if your local store has any you can usually check out the exact item you plan to buy in store.
Another option, though the most expensive, is to go with a manufacture's own refurbished units. Both Apple and Google list refurbs for a couple percent off directly on their websites.
