Sometime in the mid-90s, someone I worked with introduced me to Nesticle. I hadn't even considered the possibility of emulating game consoles on PCs before then, so it was quite a revelation. I took to that scene quickly. Most of the games in this collection worked on early emulators, with one frustrating exception being the best 8-bit Castlevania--Dracula's Curse (III). With its custom mapper and cartridge hardware, it needed several more years of emulator development and PC-hardware improvement to finally behave properly. Better NES and SNES emulators followed, and by the early 2000s, running them at full speed with no major errors was commonplace.
Given that history, it absolutely amazes me that money can still be made on the sale of these ancient games, particularly on PC. That same emulator development makes it possible too. Who actually buys these, collectors? I can't imagine anyone much younger than me enjoying something like Haunted Castle today. More than just the technology is way outdated. The better 16-bit games still stand up, though.