for speakers/sub, perhaps 70% of their SRP when you acquired them. regardless if locally branded like US Audio
for AVR, depending on its features. if it has the current audio codecs, 4K/HDR & the likes or still future proof of its features, 80-90% of your acquired price. otherwise, sell it 50-60%.
that's how I see it, to be fair on you and the buyer. but if you really need to pass it on quickly, then by all means, sell them all half their prices.
HTH! good luck on the sale of your beloved gears, sir!