value of parts

I've been getting while the getting is good and here lately I find myself with 3 tranny options for my truck... For instance, In buying a complete mid 60s /6 and 727 solely for the /6 to v8 trans adapter plate and having recently seen someone somewhere advertise a cable shifted 727 for around $300, I made mention somewhere else that I was interested in selling that trans once separated (god forbid not here) and having recently seen a cable 727 listed somewhere for sale as a core around $300,
I had gotten a PM there asking about said trans and how much.... I shot out a figure of $150. What was the reply? "If nobody else takes it 'my buddy ' will give$75"... I already quoted 1/2 of what the last one I saw listed and got offered 1/2 of what I threw out there. Dead nuts typical every damn time I offer something for sale.... I'm sick n tired of those results. I'm not rich, try to give someone a deal already to get more to put back in my project, and get lowballed even worse. Now if I were looking? Id have to pay the $300 for one... If others can get the money for their stuff I don't get why I can't....