Buy the cheapest firearms parts you can. From whatever source you can.
Don't come back in here telling us what problems you're having with them, and expect us to know anything about them, or how to diagnose those problems. We won't know what to tell you, not in the least.
It's a firearm, man. The single worst thing it can do to you - even if youre only building a range toy that you never expect to employ in ANY other situation - is blow up in your face. That's pretty bad.
If you expect it to do double-duty, i.e., range toy, and I might need it to protect my house, property, and family... Wouldn't it suck if that thing let you down when you needed it to work?... That's more than "pretty bad." That's "worse..."
Don't go cheap on firearms parts. Of course, look for deals, and find the QUALITY part that you need, for the least price...
But, ONLY from reputable firearms/firearms parts vendors. FleaBay is NOT that.
Literally, your life is YOUR hands over this, so take whatever appropriate spending actions you need to. It is entirely YOUR call.
P.S. I'm not kidding about the diagnosing thing - when people have problems here, we specifically ask them what they built, in order to properly figure out what's going on with their boomsticks. That includes "what parts from what vendors," routinely. Don't be surprised when you list a bunch of from "ladiesman217" and you get some pretty nasty replies. I'm just sayin'... <lmao>