Outdoor gear forgives a lot and does not forgive moisture, which is where most of this goes wrong.
Packing it damp
Dry it fully, then leave it another day. Fabric sealed in a parcel through an Indian week arrives smelling of mould, and mould does not come out.
Not saying how it was stored
This is the question that separates usable gear from gear that looks fine. A six-year-old tent stored dry beats a two-year-old one packed away wet, and buyers who camp know it.
Not showing it assembled
A tent pitched, a stove lit, a reel turned. One minute, and it answers almost everything. Without it the item is untested.
Not counting the pieces
Poles, pegs, guy lines, inner, fly, stuff sacks, spares. Lay them out and count on camera rather than writing "complete".
Treating age as irrelevant
Coatings, elastic, foam and seals age whether or not the item was used. Say how old it is, not only how much it was used.
Posting fuel
Gas canisters and liquid fuel are restricted and most couriers refuse them. Sell the stove; say fuel is not included.
Storing poles under tension
The elastic perishes. Pad the ends too — that is where they punch through a bag.
The one that costs most
Hiding mould. It is not cosmetic, it does not come out, and a buyer who finds it has found the thing you hid.
Questions people ask
Does re-waterproofing raise the price?
A little, if you say you did it and with what.
Is a small hole a deal-breaker?
Rarely. Show it and say whether it is patched.
What about binoculars?
Ask to look through them on camera, and pack them as the fragile exception. Packing outdoor gear.
More: selling outdoor gear, what it is worth.