I have sold (traded) a number of radio and radio gear to HRO and others over the years. Also, have sold (traded) a number of firearms to other stores. Here is what my experience has been.
First, determine what the item would SELL for used. Look at HRO used equipment list and other places to arrive at an average price for what your equipment (in your condition) is being sold for.
Then, figure that HRO or other company will offer about 60% (50% to 70%) of that price. That allows them to take in, check condition, and resale and still make their markup to keep the lights on and the doors open.
As an example lets look a radio XYZ selling new for 800. The used price would probably be about 600. And your trade in value somewhere between 300 - 420, with 360 being average.
You can always make more by selling it yourself. But then YOU need to advertise it, wait for someone to become interested, and meet your price. It sometimes takes a while for all that to transpire.
Selling it to HRO for less $$$ buys you speed (they buy it right away) and convenience.
Anyway, that's the view from Poquoson.
W4HRL
Poquoson VA