Cash is king. Some people's "way of thinking" you can buy it for $400 but if you want to trade then I need $500 in trade value. Mostly because they would rather sail it for cash me personally I think a USED stock glock with all mags, box n docs, is worth about $350 cash. Trade value to me would also be $350 worth. I'm a rare strange breed tho
The only possible clue is that your (the seller) assumes that the other party will over inflate their value when negotiating a trade.
This however creates a self fulfilling prophecy , you inflate the value of trade over sell which in turn causes the other party to do the same.....
To your point its worth what its worth to any given person.
It's like buying a used car and having a trade in. Numbers are just numbers until property changes hands, at which point numbers are still just numbers. If both parties are happy, everything is good.