I picked up both a refurb Spark Fly More combo and a refurb Phantom 4 Adv from the official DJI Store on newegg.com (they tend to have the best sales pricing there, might be a good sale this 4th of July if you are US based), Spark package was around US$320 (US$280 after a bonus credit I had) & P4 Adv was around US$800. I use both commercially and have not had any problems with either quad. And the P4 has hundreds of flights on it!
Buying refurbs is a great way to save money! But as mentioned earlier, sometimes the problems don't get fixed so you need to test a refurb out thoroughly when you get it (even features you may not normally use, as some day you might need them).
DJI is pretty good about the warranty on their refurbs, giving you the same as new, and they offered me the care package when I first registered the quads, just the same as with a new unit. Not all companies are so generous with their refurb warranties!
I would not hesitate to purchase another refurbished DJI product, especially over buying an item used from someone else!
James