how haven't all these big and trusted UGLs went out of business?
They aren't as big and trusted as you think.
Economic logic says people will switch to a cheaper source as soon as it's available and sells products of the same quality as the expensive sources do.
Look at the biggest thread with the most traffic in the underground. It's the cheapest source.
Don't try to be extra smart, just buy from the cheapest source where most people buy their stuff. There is a high chance it's the best source, or at least not any worse than the overpriced shit you buy elsewhere.