I don't think you can stereotype all eBayers as the same type of parts quality.
Consider supplier who sells items as surplus or company liquidators. Even with that however, without a clear way to trace the supply chain, you risk buying counterfeit parts. The worse that can happen is the part doesn't work, but potentially worse is that it works for a while but they are from factory quality rejects, and these items are either substandard specifications or other defects so they fail prematurely or do not live up to the spec sheet.
One safe way to buy outside the supply chain is to actually retest the part for its supposed specifications. You needn't check every specification, but you could check its tolerances, frequency response, etc., and assume if it passes key functions, they it probably has the others too. If you buy what appears to be the same brand, not a mix of different makers, you can actually destructively test a few samples to see if it lives up the the maximum rating.