Facebook marketplace is an option for local shopping. There are also groups like “Backpacking Gear Flea Market” for used gear shopping!
This online shop sells lightly-used outdoor gear. If you’re located in Vermont, you can go check them out in person!
You can purchase gear from here knowing it’s been inspected by an REI employee.
This is an online marketplace for used gear. Sort of similar to Facebook Marketplace, but specific to outdoor clothes and gear!
Used item websites will typically sell used outdoor gear as well.
Depending on where you live, there might be a resale outdoor gear store nearby. Whenever I travel West, I see these everywhere! I have found awesome deals in these stores.