In my experience the random stuff I buy off of whatever $Auction_Site seems identical to what I buy from $Local_Vendor, but with no QA or packaging. The defect/failure rate is higher and it takes a lot more time to arrive, but the cost is significantly less.

It's possible that there are more counterfeits, but I don't think the stuff off of $Auction_Site is likely (>50% chance) to be a counterfeit just because it's from $Auction_Site.

If it's a chip that you can search for in a product description it has a high enough profile to be an SZ fake. See this example of a fake signal gen IC that almost works properly:

https://www.youtube.com/watch?v=02XtneCHnDA

Add in a price that's too good to be true and the odds go up.

I can see that. I've purchased several laptop batteries that likely used old cells pulled from another laptop or whatever. At the same time, I don't mind that the runtime is a 1/2 to 3/4 of a new battery when the price is 1/5 of a new battery.