When selling a used airgun ...

Used is used and is NEVER worth 90% of the retail value unless they are in high demand and short supply. Even then it better be in LNIB condition.
For a used gun in a good market 75% would be reasonable in very good condition.
Well the market sucks right now. The economy has slowed and is showing signs of potential worsening. Anyone with half a brain can see that. Folks are hanging onto their money right now.
There is also market saturation. Some of the manufacturers have absolutely flooded the market with new guns over the last 4-5 years. The growth of this sport seems to have slowed a bit, maybe quite a bit compared to last year.
I think a lot of PB shooters got into this and decided they like the power and reach of their PB’s better.
Air gunning is a niche sport and while we all think there are tons of us and others coming on board, we remain a very small market compared to other sports. It don’t take much to flood this market.
As for pricing on this forum for used guns? People can list their wares for whatever they want to. Nobody is forced to buy or even look at the listing for that matter. Fact is the air gun market is slow even for new guns right now. Low demand dictates low prices because supply is very high right now.