I've bought used equipment from B&H before. Smooth experience and they used to not charge online sales tax back in the day
The Fred Miranda forums are a good spot.
eBay, bayBee
My main camera and all the lenses for it were purchased used from mpb.com, and I had no complaints!
Mpb, and eBay mostly
KEF but it stings as a Canadian.
Roberts Camera via eBay.
Never private. Too many scumbags out there trying to unload broken crap. Kijiji and the likes has completely trashed any and all faith I ever had in humanity.
I bought my last lens used on the "national" auction site for my country, since Ebay isn't that big for our domestic market. When I buy the next I'll probably get that used as well, it's so expensive with new glass. You can often have it for 60-70% of what it costs new, with like new functionality.
I bought my Canon EOS 40D via MPB and I liked their service, so no complaints there.
As everyone else said: yes, but only if I'm going to save 25% or more. Otherwise I'll buy new. Most OE warranties aren't transferrable and I would rather have the coverage if all I'm getting used is a small discount.
IMO, the OG A9 is a steal right now at $1,200 - $1,500 on MPB.
If you're just getting started, or are on a budget, a used DSLR like a D7x00 and some older glass is an excellent starting point.
All the great photos of the past were taken on old gear. There's nothing preventing a good example from continuing to take good photos today.
I've used eBay, /r/photomarket, and MPB for buying second hand gear. If you're using any BST platform like photomarket, make sure you use a payment method with buyer protection, and regardless I'd make sure to fully test and inspect the lens upon receiving it.
A friend of mine has bought cameras and lenses off Facebook Marketplace, but sorting through the scams to find a real listing can be really annoying. I've never bothered with it myself.
Used Photo Pro, Robert's camera Ebay store, and /r/photomarket is my primary
KEH is my 2nd choice
MBP is my 3rd choice
If you are in Europe, KameraStore is also quite nice (based in Finland). Bought my first DSLR with an extra lens and a few accessories for a nice price from there. Also, they inspect and test every piece of equipment (secondhand) so you have an added piece of mind.
