If it is high end gear, consider at least 60% off retail to start assuming good condition (but you might need to cut more - 70 - 80% could be realistic). For perspectives, close out for new rods (like the Sage One with warranty) being discontinued and replaced by an update were selling for 40 or 50% off by retailers. Some of the mid-range and entry level gear is harder to move. I think I recall seeing the Orvis Helios 2 at around a 40% markdown (new, with warranty) when the 3 came out.
Occasionally, there will be some special items that might go for more, like some highly sought after reels and a few special models of classic rods.
There is a Facebook group for used fly gear, but I can’t recall the name.
Also, perhaps you have a local chapter of FFI or a local high school fly fishing club. I have donated some gear to Project Healing Waters.
A friend let me pick a rod from his deceased fathers collection. That will remain a priceless and cherished gift.
Sorry for the loss of your friend.
Wow, I need you to do my shopping for me if you can find those kinds of prices. I have personally never seen a high-end reel (Tibor, Abel) depreciate to the degree you’re describing. Rods tend to devalue more, but even those do not generally get to 70 or 80 percent of retail until they’re practically antiques. The logic that because they could be acquired at 30 or even 40 percent off MSRP at close out therefore the used market is less than that might sound valid, but is not at all supported by reality. That would mean a used Scott Meridian ($865 MSRP) would now go for $250 or even less? Yeah, no. Maybe in 2030. For value products, I agree completely. Buying a Lamson reel is like buying a Dodge truck: solid product overall, but the savings on the front end tends to get you on the back end at trade in time too.
To the OP, sorry about your friend as well. Good fishing buddies are hard to find. In terms of pricing, there really isn’t a magic number in terms of percentage off original price. I would suggest making a list of items. Searching each item on eBay, and watching auctions to get a sense for the market.
A fly reel is not a fly reel, for example. Some retain value better than others. Things like clothing and flies can sometimes lose all practical value. Like anything else, supply and demand dictates the price, and make, model, and condition are going to be the key variables.
As somewhat of an aside, it seems the used market has changed a lot as the generational membership of its participants has shifted. At the risk of offending all the Millennials, let me just point out that the wading boots you bought for $200 are not worth $195 even though you “only wore them once.”