Dear lord, what's wrong with you guys? Data recovery companies will gladly pay a flat rate per drive (usually $7-$10). The very idea of paying to get rid of them boggles my mind. 400 drives is $2,800 - $4,000+ worth of inventory. Call some data recovery companies or drive wholesalers (these are people that buy drives in bulk then pick out the gems and sell them for a premium to data recovery companies). Some of these drives are worth hundreds of dollars if you have a rare one. Drive wholesalers are the least picky, but they pay far less (typically $2-$3 per drive).
I have a good relationship with a drive wholesaler. He travels all over the US and physically visits shops to buy their drives. He pays me pretty well too ($5-$6/drive). Pick up an old crappy server with a bunch of hot-swappable drive bays for $100 and let it rip. Or heck, buy a cheap SATA controller card and have ugly cables everywhere. Don't do these one at a time. It's not worth it then.
If you want even more money, list them on eBay. I can get $12-$15/drive on eBay if I sell them in lots of 10-20. I find it's too much of a pain in the butt though to deal with a bunch of different buyers, BUT you can contact buyers outside of eBay AFTER the sale is complete and ask if they're interested in buying from you on a regular basis. I can't tell you how many great business relationships I've started this way.
I agree that desktop hard drives still have their place for backup purposes, but I'd be wary of installing used drives in client's systems. I mean, I did it back in 2011 with those hard drive shortages and it worked out surprisingly well, but still...