IMHO it depends what you're after. I'd dodge anything that might be returned due to quality control issues but is still considered 'working', but things which have a clearer 'it's broken' state are safer bets.
Monitors? I wouldn't. Too much chance that they were returned due to BLB or dead pixels.
CPUs? Probably didn't overclock as much as someone hoped... I wouldn't expect to find a gem in B-grade unless it's one of 8-Pack's ex's
GPUs? Could just be they were too big for someone's case, or buyers' remorse over the price. Unless you're set on overclocking the nuts off it, probably not much risk here. If it works, I can't see a reason why it's more likely to fail than any other off-the-shelf sample.
Hard drive/motherboard/RAM/peripherals? They work, or they don't... be prepared to supply your own mounting screws, cables or adapters.