When determining the price of a gun for sale...
Cheap way - go to gunbroker, see what the average opening bid of the gun is. Average out the prices vs age and condition vs what you have, and determine a sell price set on how fast you want to sell it. If the average price you find is $1500 on what you have, you need to include shipping, FFL and gunbroker fee.
Sometimes you can find some real deals on gunbroker with someone wanting to unload something, and sometimes the prices aren't that great vs something new you could buy anywhere.
Because my FIL's gun was a unique older lever action rifle where prices vary greatly depending on the condition, I took it to a gun shop who specializes in valuing guns. See below for written appraisal
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At the end of the day, something is only worth what someone is actually willing to pay for it.