The thing is, a hard drive is already so cheap and relatively few people upgrade their own computers, so the market is small compared to an iPhone. Tons of people will take a used, last year iPhone for $400 instead of ponying up whatever a brand new one costs. But, you can get a brand new 1TB 2.5" HD for $65. Not many people would rather have a (presumably smaller) used HD for $35 instead, especially given the fragility and importance of a HD. Then you have the effort of wiping the drive completely and the concern that someone might find something if you didn't wipe it correctly...
You might get a few nibbles on Craigslist for $20, but there's no service that does this like Gazelle because there's simply not enough profit margin to make up for the low volume.