Local
- Local ham club (may have used gear they will sell cheap or even giveaway/loan to newer hams.) http://www.arrl.org/find-a-club
- Hamfests/swap meets (lots of older gear usually) http://www.arrl.org/hamfests-and-conventions-calendar
Online
- QTH (donation-based) https://swap.qth.com
- eHam (free-subscription) https://www.eham.net/classifieds
- QRZ (subscription-only for ads) https://forums.qrz.com/index.php?forums/ham-radio-gear-for-sale.7/
- eBay (lots of fees, prices usually higher than other places) https://www.ebay.com
Dealers
- Associated Radio (Used) https://associatedradio.com/uitem_fulllist.php
- Ham Radio Outlet (Used) https://www.hamradio.com/used.cfm
- Ham Radio Outlet (Open box) https://www.hamradio.com/open_item.cfm
- Ham Radio Outlet (Consignment) https://www.hamradio.com/consignment.cfm
- MTC Radio (Used) https://www.mtcradio.com/used-gear/
- R & L Electronics (Used) https://www2.randl.com/index.php?main_page=usedbrand
- Universal Radio (Used) https://www.universal-radio.com/alert.html
I also made a video about this topic which you can watch below.