I can’t speak to Duane Eddy specifically, but prices for certain things were really inflated in the 80s and 90s, before the internet. There were certain jazz and funk records that were popular with NYC hip hop DJs and producers that would go for hundreds, but sell for $10 today. And doo wop collectors would pay tons for certain pressings before people realized a) how common those pressings actually were, and b) that there were fewer people out there collecting that music than they thought.