Your pricing is just about right though. For vintage collectable items (things that were good for a reason in their time), they can hold value reasonably well.
A $300 1970s receiver can still be found for $100 or more these days.
But with so much audio out there, i've experienced on average a 10-30% drop in price from retail for a used item within the first few years. Something older than 5 years i won't often consider a price higher than 60% of its original retail.
Price is also very much affected by condition of the unit.
"Those who preach the myths of audio are ignorant of truth."