I believe that one of the reasons people invoke "trade value" gimickry is because it is so vexing and invariably confuses everyone. From a sales point a confused buyer is less likely to act rationally (with reason) and therefore is more likely to make an a bad deal.
As someone else pointed out earlier in the tread, when people start talking trade value, just get up and walk out.
"Socialized Medicine is the Keystone to the Arch of the Socialist State.” -Vladimir Lenin
"I predict future happiness for Americans if they can prevent the government from wasting the labors of the people under the pretense of taking care of them." - Thomas Jefferson (An early warning to Obama care)