Look, saying what do you think of 'knockoff' parts or 'ebay' parts is ambiguous; same as asking what we think of 'legit' parts. At the end of the day, everything is originating in a factory (most likely in China) with similar or same materials; what it comes down to is R&D and who did the design/legwork. I can tell you that the two (going on 3 now) 'knockoff'/'ebay' headers I bought for $50 shipped are great. I have not had a single weld crack and they have taken abuse, winter driving/summer driving, track days, you name it. That's more than a lot of 'legit' header owners can say. However there are other ebay parts (and 'legit parts') that are complete shit. You have to approach this on a seller-by-seller/part-by-part-basis. For example. OBX makes shit parts.. but their TBs work fine.. their GEN 1 LSDs work fine.
It comes down to the type of parts too. If your TB breaks what else takes a shit? If your LSD breaks what else takes a shit? If a connecting rod lets go though... If your wastegate creeps boost...
Things to consider. A knockoff turbo might blow the seals and leak some oil (been there, done that), but the likelihood of it coming apart completely is nill. I have had a legit Garrett turbo completely disassemble itself and grenade with 0 miles on it. The self-rebuild kits are $35 shipped on ebay. On a budget build.. the turbo might feasibly be something you can cheap out on.
That being said, every single part on my car was purchased new and is of a reputable manufacturer; so take that for what it is worth as well.