Typically these (and the ones on eBay) are used data center drives that have started to show signs of failure, but are not actually failed. Sometimes it’s not even that, they have just reached a certain time in service and been cycled out. For a home user these are a pretty good deal because they’re enterprise quality drives that still have useful life left, and there’s no way you’re going to load them to the level they were designed to handle so they will probably last you several years.

The risk on these failing unexpectedly is higher because you don’t really know how they were used previously. Run SMART tests on any that you buy as soon as you get them. As long as you’re building some kind of redundancy into your array, you should be fine.