I disagree. Buying for cash and trading for cash are the same thing.
I believe you're wrong there if the objective by the seller is to put cash in his pocket ultimately. The seller is taking another risk by accepting a trade; hence he wants a higher value of the trade item(s) as compared to the cash value he has placed on the gun.
It would be perfectly understandable if the seller said $500 for my gun cash or $500 +3% with credit card. This is done all the time at gunshops. The seller simply wants to minimize his risk when considering a trade for his $500 gun. Assuming the seller's objective is cash in his pocket that he can spend... he wants more estimated value on a trade for his firearm. He can't buy groceries with a gun valued at $575 until he takes the time to sell it which might take months.
Would it be more understandable if the seller said $500 cash or trade + $75?? Dealers at gun shows do that kind of deal all the time. Essentially they are getting their "profit" in cash before trying to sell the higher valued gun.
The problem there is that if it is a regular FFL dealer, the used gun price on the $575 gun is probably more like $900 to make the deal work.